Red planaria as stated by others. People saying they can crowd out corals, I don’t believe that’s correct. Perhaps in massively overpopulated conditions but many and I mean many people just deal with them in their systems. They don’t bother coral, especially hard corals. They tend to rest on euphyllia, softies and leathers but they don’t seem to restrict growth of coral. I’ve had them for years and they haven’t been a problem. They are almost as common as asternia and aptasia. Flatworm exit is more likely going to cause problems than the worms themselves. If they aren’t in your display you could disconnect your refugium and treat it, otherwise you can just leave them be if they aren’t bothering you.