Hard to tell, does it look flakey? Or is it hard encrusted on the rock, if so thay jusy might be coraline buddy wich is good. If its comes off or looks slimey thats the begining of a cyano outbreak, bad news might need to get more water movement and do a big water change possibly get chemi-clean.
Unfortunately, it's not coralline algae which I am unable to grow consistently if my life depended on it. The red algae comes off easily. With that said, it looks better than the rest of my rocks. Is there a danger in letting it grow.
looks like cyano. look at your phos levels and nitrates there are also some snails and crabs that will help eat it as well as cooper band butterfly. big and often water changes are very helpful you may also want to make sure your rodi water is at zero
Can you remove that piece of rock without disturbing the aquarium, if you let the cyano bacteria gets out of hand it can grow over corals and suffocate them
What wave maker/power head system u using.. I'm currently using a gyre and haven't got any cyno which I usualy do on sand bed and rock as u have but since u switch to wave makers and now the gyre I guess the water movement keeps them clean.. Try turning up your water movement or try new pumps