Mold or a fungus - likely harmless to the plant - can't say what it's impact on humans would be.

Al

Its likely salt leaching from the clay.

No, thats definitely fungal. Scrape it off, then rub the top of the pot down with a rag soaked in a light bleach solution, taking care not to get any in the soil or on the plant. Also, you are probably keeping the soil too moist. Ease up on the watering.

Still going with salt leaching. See how the top of that terra cotta is eroding? It was not properly fired, absorbs salt from the soil and transports it to the rim as the water flows up the pot and then the water evaporates leaving the salt behind.

an article https://homeguides.sfgate.com/terra-cotta-pots-salt-damage-103369.html

I’m also in the leaching salts camp. I’ve seen it often on clay pots. If you scrape it off what does it smell like? Not mushroomy is my guess.

btw the plants need a lot more light. They’re stretching for it.

Interesting! I think you’re right about the salt. That makes sense the way it was coming up from inside the pot. I never would have thought of that! My husband thought it was some weird mold and made me throw it out. Thanks for the feedback!

I have seen salt deposits on my own pots, but they never looked... fuzzy. It looked like fine sand glued to the pot, a true pain to scrape off, but it didnt look like that. Maybe it was growing large crystalline formations, but what kind of liquid-rock water and how much time would produce those kinds of deposits?

If the soil was fertilized there would be quite a bit of salt and if the pot was watered often and the condition just right the crystal growth could be large.

I suppose its possible. Maybe the otiginal poster could tell us if it was soft and fluffy or hard and gritty, and that would solve the mystery for us.

I have seen this phenomenon many times and it was salts. Even the fluffy stuff.

I didn’t touch it, not knowing what it was, but it appeared to be very soft and fuzzy. Very fine and fragile, not granular at all.

I had an experience that involved some gorgeous drip-glaze ceramic pots in 4 size sets I discovered at a Big Lots Stire. Sizes in the set ranged from about a quart in volume to maybe 2 gallons. I bought 10 sets (40 pots) on behalf of the San Toshi Bonsai Society in early spring. We also purchased 3 boxes (60 plants) of assorted dwarf conifers from Iseli Nursery, located in Oregon. We drilled holes in the bottom of the pots and transplanted the plants into the pots using the gritty mix so they would be well established in June for our fund-raiser sale at a local art fair. We had single and multi-plant plantings, as you would think would be the case if we had to fit 60 plants into 40 pots.

The plants were well-established by the time of the art fair and were a hit with the patrons in spite of the fairly significant outlay. As I remember, we had about 4 plantings of various sizes that didn't sell. I took those home and put them on my grow benches to tend/prune them so we could sell them the following summer.

Unfortunately, by August the plants I was tending were all in decline, the ceramic glaze on the pots had begun to craze, and at each of the cracks in the crazing pattern there was a growth of 'something' that looked exactly like the OP showed in the image (s)he provided.

Since I had about 150 other conifers covering at least 20 species on the grow bench right next to the struggling plants, it didn't seem appropriate to assign the cause of the decline, which BTW ended in the death of the plants, to grower error - unless you consider choosing pots made with toxic materials to be my error. I have no problem accepting that responsibility, even though I had no reason to suspect that bowls meant to serve as a vessels to hold food for human consumption would be made with toxic ingredients that could be expected to leach into the bowls' contents. The bowls were made in China.

While I can't witness with any certainty in the case at hand, I do strongly believe that in the case of the bowls I unfortunately discovered, one or more of the ingredients that were mixed together and formed into bowls before firing were, at a minimum, phytotoxic (poisonous to plants) and at the root of the weird formations that formed at the cracks in the bowls' finish. Remember, because of the soils I use and my watering habits, a build-up of dissolved solids in the soil is dubious in the extreme as causal of the growth formations I saw.

Finally, as I remember, I had a strong sense these 'formations' were not organic in nature and were the outgrowth of the chemical formations formed that water as it evaporated left behind.

Al

A crazing glaze happens when the clay and the glaze doesn't fit. Alot of pottery has crazing most won't kill you or your plants. The crazing could of always been there but only over time did it show up as moisture and dirt seeped into the crazing. If the clay was not fully vetrified during firing the clay would have absorded the salts in the dirt and could leach all the way to the outer surface of the pot. The pot could have absorbed seawater salt thru the crazing on the trip from China if the container had gotten wet. I can't say that the leaching killed your plants cause i have many a store bought pot leaching salts and the plants seem fine. Maybe soaking new pots in water will help leach out salt before you add plant.

the salts form crystals, that is the formations that grow.

That'sa pretty implausible list o' could'ves, and alotta reverse engineering to support conjecture, but I'll leave it to others to decide.

Have a good wknd!

Al

As far as I can see the OP’s pot is terracotta and unglazed. I repeat, I’ve seen this many times and it’s always been efflorescence ie salts from the pot, not the growing medium or anything do with fertiliser. A similar bloom often appears on new brickwork. And it has no effect on growth in my experience. Perhaps it’s more common in wet climates but it’s a common phenomenon on cheap quality clay pots and new brick walls here. Image