Yes, PROSCENIUMS is a Scrabble word!

Letter values in Scrabble US: P 3, R 1, O 1, S 1, C 3, E 1, N 1, I 1, U 1, M 3, S 1.

Is it a valid word?

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

the part of a modern theater stage between the curtain and the orchestra (i.e., in front of the curtain)
the wall that separates the stage from the auditorium in a modern theater

Also: proscenium wall.