EDIT: PLEASE DISREGARD THIS ENTIRE POST LOL... I READ "IT IS SQUISHY" 
Unless you wanna read up on flatworms lol...
Looks like a chiton but if the body is squishy (chitons have hard shells) I'd go with Polyclad Flatworm - they come in an incredible array of colours and sizes
Please read down in this link to learn more
http://www.seaslugforum.net/showall.cfm?base=flatworm