Need advice on invasive something...

[Phormium]

My 120 gal reef tank has been active and problem-free (knock on wood) for at least 5 years. I haven’t added anything new in a long time but the last fish were in 6 weeks of QT and the last corals were inspected and dipped. Ages ago, I thought I had an Aiptasia infection (as waaaay back at one point near the start I did) because I saw tiny polyps, with tiny tubes growing on the rocks. They’d retract into their tubes if you came close and they were mainly about 1/4” tall, brownish-green generally. I snapped off some of the smaller pieces of rock they grew on and took out some of the “clumps”. The clump would dry out leaving a small cluster of empty tubes. Because that didn’t seem to match Aiptasia, I scrubbed them from the rocks over and over again (and, maybe naively turned on the water polisher) and they eventually gave up and disappeared. But now they are back and more aggressive than before and have popped up all over. I need help on determining what they are and what I might need to do, if anything, about them. Any ideas?

[Dearth]

Pictures would help in positive IDs to better give you an answer

[Phormium]

Thanks, I’m going to try but they are tiny. I do have a dry clump that I can photo. I’ll see what my phone camera can pick up...

[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.