I had those, I think they came in on a zoa or acan frag. Maybe still do have some, but haven’t see in awhile. I described them as tiny Oreos. I couldn’t really trace where they came from, and I don’t have any SPS. I added a mandarin and the population has declined but I can’t say whether he was the reason.
I had a thread about it something like “tiny black crawlies” or such. No real answers found. I didn’t lose any coral from them...