Yes, HAIRBALL is a Scrabble word!

Letter values in Scrabble US: H 4, A 1, I 1, R 1, B 3, A 1, L 1, L 1.

Is it a valid word?

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

a compact mass of hair that forms in the alimentary canal (especially in the stomach of animals as a result of licking fur)

Also: hair ball.

Not playable in Scrabble US: TRICHOBEZOAR.