Yes, DICTUMS is a Scrabble word!

Letter values in Scrabble US: D 2, I 1, C 3, T 1, U 1, M 3, S 1.

Is it a valid word?

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

an authoritative declaration
an opinion voiced by a judge on a point of law not directly bearing on the case in question and therefore not binding

Also: say-so, obiter dictum.