It's a polyclad flat worm. If you see this enormous flatworm in your tank, siphon it out. It is a predatory flatworm, and its food is often clam flesh. Their pattern is designed to act as a camoflage to match the mantle on a clam, so it can park on the animal as it consumes it. Not Reef-safe.
http://www.melevsreef.com/id/flatworm.html