When we get pests in our systems, it sure can be a pain. I spent a year and a half battling vermetid snails in my 55 reef. I didn't see them when they came in and started reproducing...spread around the tank and were a pretty big pain.
Thankfully, if those colonial hydroids are simply on one rock it can simply be removed from the system to solve the problem.
They can be addressed in a similar fashion to aiptasia/majano where a product like aiptasia-x or joe's juice can manage them effectively. I just added a ton of coral into my system. One of which just showed a large amount of aiptasia on the rock itself. They weren't showing themselves in the store, but some are pretty big. Will be an easy fix with aiptasia-x, which is my preferred product to use.