Yup, that is a polyclad flatworm..
Here's one in found in my tank quite a while ago:
they are obligate predators of tridacna clams. If you look at the pattern it looks simiar to the spotting of many species of clams mantles.
They tend to come out once the flow has been off for a bit. Normally the respire with the water movement over their bodies, but when the flow in the tank is shut off they have to come out and move to keep water flowing over them to breathe.. Mine came out one night when i had my previous tank and the hob overflow had lost siphon and the tank was on the verge of overflowing. My wife called me and i told her to just shut down power to the return as i was due to be home from work shortly. Found it moving along the glass and sand.. I had a clam die unexpectedly about 2 weeks beforehand.