Yes, HAG is a Scrabble word!

Letter values in Scrabble US: H 4, A 1, G 2.

Is it a valid word?

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

an ugly evil-looking old woman
eellike cyclostome having a tongue with horny teeth in a round mouth surrounded by eight tentacles; feeds on dead or trapped fishes by boring into their bodies

Also: slime eels.