Sorry, I should have said moss, not algae! Well, I think it might possibly be detrimental, though I'm not sure. It can form a layer across the soil surface that I imagine might possibly prevent a cotyledon from pushing through... ? Pots with significant moss growth are usually ones where germination is really, really slow or has failed... so it could be I'm mixing up correlation (presence of moss in "dud" pots) with causality (the moss actually having something to do with the failure of the seeds). In short, I don't know...
