Are you using 1/2 perlite & 1/2 soil mix? Do you have a small fan blowing over them to keep the air circulating? Is the soil damp? Or wet? Or dry? Rosie

Too much fertilizer - you are getting fertilizer salt exuded by plant.

Cosmetic damage, no harm done.

Rinse the plant and the soil with tepid water. Let it sit on a stack of newspapers to drain.

The plant is sitting too deep After rinsing it will sink even deeper. Pop it out of the pot, add some perlite on the bottom, pop it back.

Now - fertilizer - if you use a fresh commercial soil - like Promix BX - it comes preloaded with fertilizer. Keep the plant for a month on water. Restart fertilization - 1/8 teaspoon per gallon of water. Young plant doesn't need as much fertilizer as a full grown one - so it absorbs it - and then sheds through the center leaves. Additional factors - less humidity - less fertilizer - solution evaporates faster. If your fertilizer is more like 20:20:20 - 1/8 of a teaspoon, if it is more like 10:10:10 - 1/4 of a teaspoon. Underfertilizing - is less damaging than overfertilizing.

The soil is pretty wet. I do use 1/2 MiracleGro AF mix and 1/2 perlite. No fan though. Just set them in front of the southern window.

For now I took all the plants off the wick to let soil dry up. For the orange ones I took them out and used the paper towel to dry out the root and replanted with one strand of yarn. I am also in the process of replacing all the wick to one strand (1/4 of the yarn).

i do not think I fertilized the plants. But can't be sure because I had a pot of water with fertilizer I might have used for some of the plants.

Good to know the plants are not sick!

Nope. Purely cosmetic.

The MG soil is preloaded with a slow release fertilizer for 2 months - and with more humid soil - it gets released faster.

The first is a water mark and nothing else. The second is fertilizer build up like Irina said.