Could you confirm a list of water parameters and also age of the tank as it looks fairly new ? Has it been cycled etc as I cant see any fish or anything ?
That does not look like algae. Does it blow off the rocks easy with a turkey blaster ?
The reason we are asking for parameters is dinoflagellates thrive in low nutrient systems. You will need to test for nitrate and phosphate to get a handle on algae. You’ll need to test for alkalinity , calcium and magnesium if your keeping corals
Might be dino's. Do you have a toy microscope or anything like that laying around. Because the easiest way to identify it is under a microscope. Although there are other methods too. My guess is dino's or diatoms.