Grab a spoon and scrape it off. It'll only be on the very surface of the soil. Recover with fresh stuff if you need to. It will not harm large plants but is sometimes of concern for small Sarr and Drosera seedlings because it can grow over them. If you fertilize, fertilizing less will stop the algae too.
If I scrape it off won't it just grow back? I don't use any fertilizer.
It normally grows if their are added minerals, these can come from fertilising, within your water or just with the soils, good air flow lessens it, but really you have to repot or cover it with some more soil, it thrives on peat and spag moss, but a small top layer of sand or perlite (1-3cm) should slow it right down.
Yeah, just go ahead and change out the top of the soil.
Formerly known as WaterKirby.
This is probably one of the Nostoc algae. You can usually just pop it with something sharp like a toothpick and it dries out. As said before before by many just scoop it out with the top layer of your media and replace it. Propagules of algae are everywhere so re-occurrence is inevitable.
Grand Hotel... always the same. People come, people go. Nothing ever happens.