It looks like a flatworm that someone else had posted a thread about a while back, and if I remember correctly, they came to the conclusion that its a clam eating flatworm. I wish I could find the thread...
"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."