Need advice on invasive something...

[Phormium]

Stupid phone camera. Anyway here’s one of the dry clump and one in the tank.

[shrimp]

Hydroids
Hard to get rid off. I ended up selling my live rock.

[Phormium]

Hmmm, seems like very bad news. Apart from dismantling, would anything else help? Maybe not an iron-clad solution like boiling all my rock, but how about something like smothering them in epoxy, or just keep scrubbing/tweezing every week during maintenance, reduce feeding, anything else maybe...???

[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

[Frogger]

Wouldn't recommend ever boiling your rock. Too many people have ended up in the hospital doing that.

Colonial hydroids are no big deal. Their sting doesn't seem to harm any of my corals. And they eventually get knocked back by active healthy coral growth.

I have had them in my tank for 20+ years and they never amounted to much. I can still find them if I look hard enough.

If you really want to get rid of them and you have tons of time and patience on hand you can bleach your rocks followed by a Muriatic acid bath (outside only) and you can start fresh with completely dead rock. However you will have to recycle your tank and likely go through the ugly phases for the next few months.

Any time you buy a coral with a bit of rock, a bit of exposed dead skeleton, plug or a snail, hermit crab you risk adding a nasty.

There are many many nasties that are way worse then hydroids. I would reserve my fight for something far worse.