Sometimes green on the soil is an indication that the soil is too acidic, but if yours seems slimy and you think you over watered it is probably a mold.

DO NOT spray with bleach. If you have a humidity cover on them, remove it. Get a fan blowing on them and sprinkle the top with cinnamon.

If it's just green from acidity then you can sweeten the soil a bit with crushed egg shell. It will take a while to work because the shell material has to wash down into the soil with watering.

When watering, wait until the soil is dry at LEAST half way down the pot, and then set your pots in a shallow container with water and then wait until the soil darkens on top a bit and remove it from the water. Now you know the soil is wet right through and it can be a week or more before you have to do it again.

I had a bit of green 'stuff' on my soil line. It was not slimy, looked more like really bright moss. This was showing when teh plants were very young still. I can't say if we have the same green 'stuff' but mine was harmless. I did do what oilpainter suggested with increasing airflow but it did not go away. It dissapeared when my petunias got larger and blocked the light to the top of the soil line.

This had no effect on my petunias, they are now about 8" tall and 24" across.

Keriann~

It's likely algae and shouldn't hurt your seedlings. Do not spray with bleach, the ph of household bleach is somewhere around 11 and would be fatal to plants. Water less, increase air circulation around your trays. You must be close to being able to put your seedlings outdoors to begin to harden them off, just that step alone should resolve the problem.