wimszeewater 24 december 2015 Bewerken Cyanobacterium (“rode flap”) quickly overgrowing a patch of Star polyps

One colony of star polyps withered away and was then burried under a flap of cyanobacterium.

My guess is the bacteria fed on the remains. See pictures.
Removed them during siphoning, replacing 25 liters (20%) + removing of the green algae.