Yes, LEXEMES is a Scrabble word!

Letter values in Scrabble US: L 1, E 1, X 8, E 1, M 3, E 1, S 1.

Is it a valid word?

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

a minimal unit (as a word or stem) in the lexicon of a language; `go' and `went' and `gone' and `going' are all members of the English lexeme `go'