Caroline, did you sterilize the soil before sowing? I think that helps prevent it. Not scientific, just a personal observation. I also remove the humidity dome after germination and run a fan on them at least for a few hours a day. (my house is too cold in winter to run a fan all the time!)
I think algae usually results from too much water and/or not enough light. Did you use a seed mix? In my experience I find less algae with regular potting mix than seeding mix. I never use the stuff anymore. I have rarely had a speck of algae but it happens a lot with my wintersown stuff (always wet and not much natural light...)
Bottom watering , rather than from above, might help too.
Karen
