I have some of these little buggers in my tank as well. The patches around my shrooms and buttons seem to keep them from opening up completely. A conversation on my club forum suggested using kalk paste made from pickling lime to get rid of them. Here is the main points of the suggestions, posted by DonJasper...
" I've found that Joe's Juice works, if you keep applying it. At like ~$10/bottle - that's faint praise. (For apastia)
Kalk paste is messy looking. But at like ~$2 / lb for pickling lime - it's pretty cost effective. And it kills everything.
Simple removal works pretty well, and is the most cost effective of any solution.
This is what I would do. Focus on the rocks that can be moved with a minimum of effort. I'd gently remove them from my tank, whack off the infected chunk and replace the 'clean' rock. Taking baby steps I'd do "one" rock a day. I'd consider grinding away at the infection until I reach dry rock (to make sure I got it all). You can also peel hydroids away, but knowing me I'd miss a small patch and have to do it again.
Now on the more difficult rocks. I'd treat them in place, and smother a small area in kalk paste. Small as in half-dollar sized spot. I've got a syringe/tubing setup to do that. Mixup a batch of pickling lime / water to the consistancy of paste, and slather it on the 'weeds'. It's not real sticky underwater, and it takes a little practice to not spread dripping blobs of the stuff when working with vertical surfaces.
The trick with kalk paste is that it has a ph of like 12. Very caustic - it burns things on contact (which is good), but when it comes in contact with the water all the Alk/Ca gets sucked out and it forms a surface layer of nothing. Two effects. One is that to really make it work you have to apply is such that it balloons out - and the inside of your dollop of kalk paste has the nasty ph of 12. I like to think of it as a cookie press (Real men do bake cookies). And even if the ph doesn't get them - they'll be encased in a white rock-like coffin. The second effect is that the stabbing dollops of kalk paste is giving your tank an Alk/Ca boost. Which in moderation is a good thing. So a small area every second day or so.
You could remove the difficult rocks - but the resulting moving frenzy / dust storm will cause a micro-cycle in the tank. Anything that's not robustly happy make slip the surly bonds of earth. I'd expect my tank to need a couple of weeks to settle down after such an event. Personnally I'd rather watch ugly white blobs on my rocks for awhile. If you really hate them - they'll have turns to soft rocks after a week or so and you can remove them. But I'd give them a month - just to make sure everything is dead.
"ATTACK PLAN".
Smother them in kalk paste."
