Looks like a polyclad flatworm.
Edit: Did you dip the corals you received? Did this guy fall off during the dip? Either way, if it’s a polyclad it’s no good, but you should dip/check for others.
Unfortunately, I didn't dip, I just emptied them into another container with all the water from the bags. It fell off during that. At first, I just thought it was some algae. Then, after I was done transferring, I noticed it was moving around in the bottom of the bucket ugh.