Helooks like a nudibranc. Probably from the genus Dermatobranchus. Possibly Dermatobranchus rodmani - depends on what kinds of markings he has.
Flat worms are just that, flat. Very flat and wide in relation to their length. Some have a slight dorsal ridge, bit it's only visible on the really big ones.
Your little guy also has clearly delineated mouth parts that look very nuidi like. If you only see him on crusing the glass he probably eats algae.