Yes, SUBSTRATUM 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.

Is it a valid word?

SUBSTRATUM is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and 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