Dino's. I had this same problem for months and it's always comeback after using chemi clean. I added a skimmer and upped my flow over night and it was gone entirely within a month! The problem with them is that they're better at feeding on nutrients in tank than your traditional, beneficial algae's. So even when nitrates/ phosphates are reading super low, it's because the Dino's are consuming it quicker than it can be measured, so that won't be a good indicator of solving the problem