It is very small and I only see them at night. There are a few of them, look like small upside down shrimp. Also think they are eating my polyups. Too small to photograph.
These sound like copepods, or some other type of small crustacean. They are generally harmless (though isopods are sometimes known to become parisitic to fish, they are relatively rare). Unlikely to be eating your coral, if these are the ones in your tank.
Do some searching on Google for copepod and see if they look anything like the ones you have.