Yes, STEMMERS is a Scrabble word!

Letter values in Scrabble US: S 1, T 1, E 1, M 3, M 3, E 1, R 1, S 1.

Is it a valid word?

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

a worker who strips the stems from moistened tobacco leaves and binds the leaves together into books
a worker who makes or applies stems for artificial flowers
an algorithm for removing inflectional and derivational endings in order to reduce word forms to a common stem
a miner's tamping bar for ramming packing in over a blasting charge
a device for removing stems from fruit (as from grapes or apples)

Also: stemming algorithm.