Growing Problem

[Tonyzoo]

There are these small circular pods rapidly multiplying in my system. They are mostly green with bright blue spots on there back. They don't appear to eat coral, but they are starting to compete for space. It's hard for me to tell if this a type of algae, flatworm or nudibranch. If I had to guess I would say it's algae of some kind, but the tangs, blennies and snails won't touch it. I'm going to add emerald crabs soon

Any idea what they might be or how to get rid of them?