Some of my seedlings have a green algae on top of the compost. It seems to be on any of the pots that have been in propagators. Any ideas why this is happening? Is it because they aren't getting enough light and can i stop the spread?
They're too wet. Stop watering from the top, watering only from the bottom, and ease up on the watering generally. Also, increase the air flow - the atmosphere is probably too humid. Once your seeds have germinated you can take the top off the propagator in the daytime, and at night if it's not cold.
A good tip to reduce algae /liverworts/ moss is to sow the seed in compost as usual but cover them with fine vermiculite rather than compost. Algae live off the nutrients in the compost - vermiculite is nutrient free. This is also a good way of avoiding algae on cuttings that take a long time to root, such as camellias, rhododendrons etc.. I am not sure whether the algae do any harm, but it can appear alarming.