Yes, you can bring in all sorts of things in any bag of mix/compost/mulch. There is no expectation of any sort of sterility. Since you get moss/algae after a week of no water, and that mix's surface is still wet, you probably have to change the immediate environment that allows this stuff to flourish.

Don't panic, that doesn't look severe and isn't likely to hurt anything. How is your air movement in your basement, do you have a fan or furnace vent or something to combat stillness?

You can take a plastic fork, garden tool, chopstick and just fluff up the very surface a little. And increase some air movement if you can, your seedlings will thank you for it by growing a little tougher anyway (it increases the thickness of cells in leaves).

Algae are plants, although very rudimentary plants, not a disease or something horrific. We don't especially want them on our sowing medium tops where they can create a crust, even compete for some nutrients, but a bit of algae shouldn't affect your seedlings. Its likely to occur when ambient air is moist as well as the soil....is your basement dampish as well as cool?

Mor is exactly right. All you really need to do is keep the crust stirred up a bit. I doubt that the medium is still wet...just looks that way. Your little seedlings will need water and fertilizer as they quickly develop their root systems.

Thanks everybody. I do have a fan running a few hours a day. Fluffy the top I will do. Good idea! The seedling look fine so far and hope they continue to grow.

Yup. Let's not blame it on the mix, whatever brand you use. Spores are in the air everywhere and it is the ambient growing conditions provided that either prevent its development OR allow it to flourish. ProMix also contains lime but likely less than you might add to your homemade mix. So your homemade mix may simply be more alkaline in pH. And you may need to keep the fan running more than a "few hours" a day. Circulating air is always a good thing.

Dave

Thanks Dave. You are right, no blame on the mix. Ambient growing condition is definitely far from ideal. But to fix it requires more $$ than I can justify for a casual hobby for now. Hope to find ways to improve in the future. Pro-mix is new to me and I guess I just need time to get the hang of using it.

Cool idea! That sounds totally doable. And a perfect way to further stupefy my poor clueless DH! LOL.