Yes, SCLEROTIUM is a Scrabble word!

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

Is it a valid word?

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

form genus of sterile imperfect fungi; many form sclerotia; some cause sclerotium disease in plants
compact usually dark-colored mass of hardened mycelium constituting a vegetative food-storage body in various true fungi; detaches when mature and can give rise to new growth

Also: genus sclerotium.