Yes, MODERATOR is a Scrabble word!

Letter values in Scrabble US: M 3, O 1, D 2, E 1, R 1, A 1, T 1, O 1, R 1.

Is it a valid word?

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

any substance used to slow down neutrons in nuclear reactors
in the Presbyterian church, the officer who presides over a synod or general assembly
someone who presides over a forum or debate
someone who mediates disputes and attempts to avoid violence