Answer: Mold or algae on the soil surface can be caused by overwatering, but if your seedlings are wilting when you cut back then that is not it this time. Sometimes there will be a bit of algae growth on the soil surface and it does not hurt the plants, it can develop due to the soil mix or sometimes the fertilizer you use being conducive to it over time.
Since your seedlings seem to be affected, I would check that you are watering enough to keep the soil damp like a wrung out sponge and not sopping wet or bone dry; that you do not wet the foliage when you water; that the seedlings are well thinned or in indivdual pots large enough to allow them ample root space and light; that they are getting enough light from a very bright window or supplemental fluorescent lights close to the plants; that they have good air circulation to help prevent fungal infections; that your tools and pots are clean, that you are using clean, fresh soilless mix.
Sometimes seedling will topple over due to damping off. Checking the above will help prevent that, but if you see it starting to happen you can sometimes dust the plants with cinnamon or mist them with chamomile tea (make the tea and let it cool first) and stop it from progressing.
I hope this helps you trouble shoot.
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