Soft Shell is a quest found in Epic Mickey: Power of Illusion.
Ariel will ask Mickey to find her friend Sebastian for her. Sebastian can be found in Under the Sea, near the beginning of the level on a ledge. To reach him, Mickey will need to either use a Platinum Platform sketch to leap up to the top of the pillar from the left, or bounce up from the right.
Once rescued, he will he will return to Ariel's Room, and Mickey will be rewarded with 512 E-Tickets and a Room Star.