Beard algae. Manual removal with filters off and vacuuming/ water changes might help, but I've been battling this myself and have given up on completely getting rid of it. You can spot treat with peroxide or excel that might get rid of it.
Thank you. I took the affected objects out of the tank and soaked them in a container with Peroxyde for about 10 min. That did the trick. I took off most of the algae and the few spots that were left turned a beautiful red colour!