Yes, STROMATA is a Scrabble word!

Letter values in Scrabble US: S 1, T 1, R 1, O 1, M 3, A 1, T 1, A 1.

Is it a valid word?

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

a mass of fungal tissue that has spore-bearing structures embedded in it or on it
the dense colorless framework of a chloroplast
the supporting tissue of an organ (as opposed to parenchyma)