I have the same thing. I have baby hippos in my tank and they like to wedge them selfs in tight places to feel safe...
They don't do anything to the coral besides make they close up for a little while
It won't hurt anything, like previously stated when tiny they like to squeeze into tight little spots. When one of mine was small it would sleep under the nem my clowns were sleeping in and would try to follow them around during the day but the clowns wouldn't have it. I was always worried the nem would eat the hippo but somehow it never did.