Yes, SUBSTRATUMS is a Scrabble word!

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

Is it a valid word?

SUBSTRATUMS is a valid word in the official Scrabble dictionary (US) and the UK Scrabble Dictionary, but not in Words with Friends (ENABLE).

a surface on which an organism grows or is attached
any stratum or layer lying underneath another
an indigenous language that contributes features to the language of an invading people who impose their language on the indigenous population