What is this circular object with a raised white middle thing on the back of my tank?

[Cchamberland]

This is in a 20 gallon AIO mixed reef tank. It's a little bit larger in diameter than an eraser head, and wasn't in the tank that I noticed yesterday. It's currently chilling on the back glass of the tank, hence the garbage photo. Tank is moderately stocked with primarily LPS and a few SPS. The only thing I added recently was a nudibranch for aiptasia control, and while it could be their eggs, it doesn't look like any of the reference photos I've seen.

 

[WalkerLovesTheOcean]

Looks like a type of flatworm. Maybe euphyllia eating? I would just remove it. It's not worth the risks.

 

[Cchamberland]

Thank you, that's what I thought as well. I went to remove it with some tongs and it was chalky and scraped off into a powder, so not like a worm would be removed. Now I'm thinking either snail eggs or the fastest growing spot of coralline algae ever.

 

[Rjukan]

Looks like a type of flatworm. Maybe euphyllia eating? I would just remove it. It's not worth the risks.

This is what I thought at first glance as well