High nitrates and too much light are the usual cause of unwanted algae. High levels of phosphorous in the water will also do it.
I had a bad case of black brush algae + algae that looked like yours becasue my tank was getting some direct sunlight, lights in hood were on too long and not enough water changes.
Blocking sunlight, limiting artificial light to 6 hours a day and more frequent (now weekly) water changes eliminated the problem. Zero algae now and crystal clear water ...... and zero algae on decorations.