Bleach water soak on plastic plants, decorations. Feed less, shorter light periods, less direct sunlight, phosphate remover in filter, and more frequent water changes.
If that is black beard algae, (Cyanobacteria) it is tough. Amount of light don't seem to phase it.
I some is a very low light tank, and was able to reduce it using Rid-X. The Rid -X provides a population of bacteria that compete for nutrients starving it out.
There is a sticky about its use "probiotics"
at first it was thick, above.
Same log, a few weeks after the use of Rid-X