Without a photo, it sounds like you are describing algae. It's not something that will necessarily harm your seeds or seedlings, but could be an indication your sowing mix is either slow draining or too wet.

Topping pots with grit, either chick grit or horticultural grit, may help eliminate algae growth - very few of the things I sow are without it. The soil under the grit remains moist while the grit dries out faster, and it's the top surface in light where the algae can appear.
But please check your moisture too and be sure you aren't keeping seeds and medium too wet.

Thank you very much for the feedback! Hopefully, that's what it is, and it won't harm the seeds... It did form on the cells that were relatively wetter. We will use grit in the next crop. That sounds like an excellent idea. :)

I am planting more of my seeds in a gritty mix and am happy to get away from he peat which is very good at staying too wet and growing algae. Al

A weak solution of chammomile tea sprayed onto algae works wonders. Also, very important to provide some moving air. We use an oscillating fan hooked up to our lighting timer.

Al,

Can you describe this "gritty mix" you use for seed starting? What's it made of and what kind of plants are good to use with it?

Thanks.

kerwee41 look into the container grower forum for descriptions of gritty mixes. If I use a peat based mix I will usually use a light grit cover as popularized by Ken Druse in his book "Making More Plants". Al