Yes, CESIUMS is a Scrabble word!

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

Is it a valid word?

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

a soft silver-white ductile metallic element (liquid at normal temperatures); the most electropositive and alkaline metal

Also: atomic number 55.

Not playable in Scrabble US: CS.