Algae grow in moist and light conditions so it may well be that there is too much water in the compost. I always water my seedlings by placing the tray in water, never watering from on top. That way the top remains relatively dry. I judge wether the seedlings need water by the weight of the tray.
Many thanks Dave. Much appreciated. I was wondering about this, because its happening to others on the allotment - I could understand if it was just me, but some of the other people have had their allotments for years!!!