Unknown critter

I chanced upon this baffling creature in 17m of water at the San Juan Reef dive site (off the western coast of Samal Island, Philippines). I was taking photos of an elephant ear anemone and spotted this very flat, translucent, crawling thing underneath the anemone's disc. Apparently disturbed, it crawled to the top of the anemone and continued crawling away from me. It measured approximately 3cm long and 2cm wide.