This is a bubble snail known as a Miniature Melo (Micromelo undata) which shares the same class (Gastropoda) as the nudibranchs. The oval bubble-shaped shell is thin and fragile and is actually too small to contain the soft body of the snail. Therefore, the colorful snail has to rely on swimming and camouflage to avoid being eaten. The Miniature Melo eats small toxic worms and incorporates the poison into their tissues as a defense mechanism. (Source)