There isn't much opportunity in a slugcat's life for them to relax, so they've got to take that chance when they can! This one's a fan of bubbles, and went out for a swim off the shoreline to enjoy some bubbling. Thankfully, it seems to be pretty peaceful out there right now.
The background is based on a particular screen in Shoreline, which is probably my favorite in the game... I wonder if any Rain World nerds will recognize it?