need sea slug ID

[CARM]

I found this guy in my tank recently, anyone know what kind of slug it is? He's pretty big especially when his body is fully stretched out (he's kind of ball-ish in this photo), maybe around 2.5-3"? I circled his head in case it was unclear lol

 

[Fluff]

Looking at the area you have circled, it looks like a chiton. Harmless algea grazing hitchiker. Enjoy him.