OK, I was able to take out the crabs as they were emerging at night.
I honestly don't know if the big invertebrate that I pulled out is a crab! The really tiny one with the green back looks like a crab to me. They are both very slow moving creatures. Anyway, the larger of the two was definitely growing in the brain coral for quite some time because after I took it out, it left a substantially deep hole (about 1 cm). I hope the brain coral will just grow over and close this hole up.
If anyone can help identify what I pulled out, that would be great.
..and here are the pictures:
