Slugfest

Yesterday, I spotted what I think is a slug, Nudibranch or a flatworm in my tank. It is in the center of the picture below. It is gray with 3 or 4 black circles that are white in the middle. It didn't stay in this location for very long. I have no idea where it is now. Has anybody ever seen one of these before or know what it is?



A few days after this was originally posted I spotted another slug type thing. It looks different and was bigger. It's body is more visibly segmented in what look like plates.