This is navanax, most likely Navanax inermis. As part of the opistobranch subclass, these have a reduced internalized shell. They are voracious predatory slugs which generally inhabit eelgrass beds and mudflats. They track other opistobranchs (like bubble snails) through the mudflats by following their mucus trail. When they catch up with their prey, they swallow them whole. After the snail has been digested, they excrete the whole empty shell. When disturbed, navanax produces a yellowish fluid which alerts other navanaxs to danger.