Crab question?

[seashells]

My dog woke me up a 3 am to go out....So picked up the always close flashlight & peered into tank. Lots of wiggley bug things running around. Then I see a piece of rock about the size of a nickle wiggle in a hole. Thinking I might just have found the monster that makes that Snap noise, I used a wooden skewer to pull the rock away! There it was...Just an ugly brown red eyed crab!!. This moring the hole in the rock where the crab is, is filled in with rocks. Anyone seen this type of behavior from a regular crab???