That is a polyclad flatworm. Likely Amemiyaia pacifica, though I can't be certain with these photos. Either way, you'll want to remove it and be on the look out for more / eggs, which also require manual removal. Dipping all coral, prior to introduction into your system, will aid in prevention. For control, you can look into predation, and if your system can support them, add a wrasse. Either Halichoeres sp. Or Macropharygodon sp.
I hope you found this information helpful. Happy reefing!