I'm using the PVC lightstand with shop lights as source for heat to germinate vegetable and flower seeds with great results. Planting in flats of perforated disposable pots. Green mossy stuff is growing on the top of the soil in some pots, not all of them. Is there something I can add to the water to prevent this from developing to begin with or stop it after it has already grown?
It sounds like you are having a problem with algae. The only thing you can do is to water less and keep the plants on the dry side. Another thing that will help is to water only from the bottom so the top of the media doesn't get saturated. Having lots of algae can be a problem as it may form a hard crust on top of the soil mix which will inhibit air transfer into the soil and keep the soil overly wet. If this happens, scratch the crust and break it up with whatever size utensil works for you.
Have you tried Hydrogen Peroxide? You add 1 Tbls to 1 cup of water and pour it into a spray bottle. Mist the surface every other day or so and it should clear it up. It is also good for misting all your plants once a week or so. I have noticed that the seedlings I mist once a week are doing better than the ones I am not. Don't do it more than that though because it could kill them if you over do it. Also, make sure that you are using NO MORE than 1 Tbls. I have 2 bottles; one is for my problem soil (1 Tbls/1c) and the other is for general use (2 tsp/1c).
I hope that helps.