Purple/black slug-like critter

[AltitudeAquarium]

I discovered this purple/black colored critter. As soon as the light comes on, it disappears. The aquarium is a 37 Gallon with a HOB Seachem Tidal filter. It has been running for 9 months. The rock is from my other aquarium that has been setup for over a year. In the setup I did get some live rock. However, I've never seen this critter before. Any ideas?

 

[blaxsun]

None, but let us know if you figure out what it is!

 

[Steve and his Animals]

Looks like some kind of flatworm, but I'm no expert in those.

 

[redfishbluefish]

Potentially a spotted flatworm. It also might be a nudibranch, but would have to see the head to tell.

 

[CanuckReefer]

Oooh, this is cool, or maybe not lol. Thinking Nudi, but not sure. Get us a clearer picture if you can!

 

[Uzidaisies]

My best guess is some kind of sea slug. Looks more like a hare than a nudibranch to me. But I really don’t know.

 

[redfishbluefish]

Let's get some expert eyes on this one.... #reefsquad @KJ

 

[Uzidaisies]

Oh wow, I didn’t even know about these big pretty flatworms. Are they all baddies?

 

[a.t.t.r]

Looks like a scutus snail to me. It looks like it has a hard shell under the black? Give it a gentle poke and see if there is a white shell under the black on its back. If it is a limpet the skin is actually just folded over, https://en.wikipedia.org/wiki/Scutus#/media/File:Scutus.jpg