The aquarium is 880 liters. It has been in operation for a year. This has been happening for the last month. I submit the pictures below. From coral only soft. And the fish has two tangs, two clown fish, bird fish and a small trigger niger.
Cyanobacteria.
Manual removal is important as most inverts won't touch it; siphon as much as you can off the sandbed and blow it off the rocks with a baster (siphon this out as well or catch it in a fish net)
Cyano thrives in low flow; increase flow to the areas you see it form.
Nutrient imbalances can contribute to cyano; please post your nitrate and phosphate values.
You said the tank is a year old; have you added any new dry rock lately?