its algæ, a very hard one to get kvits.Stems from not having enough co2 in the water. If you add co2 to the water, these bad algæ cant thrive s quick as the good plants can. You'd be surprized what co2 can do for your aquarium!! Look into getting a co2 set-up. By the way, that kind of algæ you show there, doesnt get eaten by any fish.