Doesn't look like diatoms to me. Diatoms grow flat on a surface, not in clumps like that.
I got some algae like that in a Dirted planted bowl I set up because excess nutrients leached out of the dirt. They were very easy to siphon out though.
Do you know your nitrate, ammonia and phosphate levels? Do you dose any fertilizers or use CO2
? And what type of lighting do you have?