Yes, DECLINING is a Scrabble word!

Letter values in Scrabble US: D 2, E 1, C 3, L 1, I 1, N 1, I 1, N 1, G 2.

Is it a valid word?

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

grow worse
refuse to accept
show unwillingness towards
grow smaller
go down
go down in value
inflect for number, gender, case, etc., "in many languages, speakers decline nouns, pronouns, and adjectives"

Also: pass up, turn down, go down.