Yes, YELLOWTAIL is a Scrabble word!

Letter values in Scrabble US: Y 4, E 1, L 1, L 1, O 1, W 4, T 1, A 1, I 1, L 1.

Is it a valid word?

YELLOWTAIL is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

superior food fish of the tropical Atlantic and Caribbean with broad yellow stripe along the sides and on the tail
game fish of southern California and Mexico having a yellow tail fin

Also: yellowtail snapper, ocyurus chrysurus, seriola dorsalis.