batangas, philippines | what type of sea slug is this?

went freediving a few days ago and saw this sort of snail (length is about 3 inches, dark purple almost brownish back w some spots and a white belly) swimming vertically. i dont have an underwater camera so i dont have footage of what i saw but attaching a video of a similar looking animal swimming in the same motion.

been wanting to find more images w it but i cant seem to bc i dont know its proper name!

any leads are appreciated, thank you! <3