Its been a while since we've had a tuesday nudibranch. This one is sometimes called the Leech headshield slug: Chelidonura hirundinina.
Its a tiny little slug which is found on sand often cruising amongst the sea grass - the turtle grass stalks above are good scale indicators.
An interesting tit-bit about this tiny species is that its distribution extends to both the caribbean and the indo-pacific making it one of the most widespread nudi's of all. This one was off the coast of St. Vincent.