I think it's probably algae like the other member said because I have the same thing right now. I just realized it was algae yesterday but the last week I was worried and for the life of me couldn't figure out what it was. I was told to cut the lights back and it should stop. In my situation it definitely can't be from fish flakes or over feeding because I don't even have any fish( currently fish less cylcling). Anyway I'm just a rook but I would start with giving them a wash and then cut the lights back.
Thanks. I'm thinking about getting a timer for the tank because I'm not always home early enough to shut it off. Plus I've heard that consistency is better for the fish. Pretty sure I'm not over feeding as I'm fairly OCD about only giving them what they can eat all up. Whatever it is the fish don't seem to be bothered. Every day they're getting more active and settled into their home. Cherry Barbs are algae eaters too so I guess they'll help a bit!