Need advice on invasive something...

[Dearth]

Hydroids are known to be difficult to remove

Here are some things to try

If you can bleach or boil your rock
Burn them off
Kalk paste all that you can get

Copperbands and peppermint shrimp have been known to eat them but generally only if nothing else is available to eat first

This is a marathon so expect a long fight it is very rare that you win this in a sprint