Looks like cyano. People say it is connected to nutrient levels, but there isn't compelling evidence that altering your nutrient levels impacts it at all.
Manualy removal followed by a 3-5 day lights out, while running some kind of good mechanical filter and storming the tank with a baster daily during the lights out has worked for a lot of people and doesn't mess with your stability.
I am not sure about nutrients it has been a while since I havent checked. I feed once a day a little cube of frozen minced jumbo shrimp follow d by very little dried pellets and artemia for my baby bengai cardinals. The tank has been running for 3 years now. I have roller filter since August 2022, I will try to vacuum the sandbed with the end of the hose in the roller filter. I have a UV running as well, and algae reactor that is growing well in the tank. I will check for nitrates and phosphates and see what are my readings other than that I am running neptune trident that shows my readings are stable. I only dose kalkwasser. Thanks for the help I will see what I can do to overcome this issue the tank looks very ugly with this on the sandbed.