Thank you for the link.
However it has lots of black hair, not like nudibranch or slug, kinda GHA but black.
Anyway I sprayed H2O2 to it. And we'll see whether it will grow back.
It is cyanobacterial blankets which create zero oxygen zones, that makes the surface turn black. This surrounded sponge within rock and is an indicator of high nitrates or phosphates.
Would make sense why peroxide cleaned it up