Yes, MUCUSES is a Scrabble word!

Letter values in Scrabble US: M 3, U 1, C 3, U 1, S 1, E 1, S 1.

Is it a valid word?

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

protective secretion of the mucus membranes; in the gut it lubricates the passage of food and protects the epithelial cells; in the nose and throat and lungs it can make it difficult for bacteria to penetrate the body through the epithelium

Also: mucous secretion.