Yes, DECEASING is a Scrabble word!

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

Is it a valid word?

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

pass from physical life and lose all bodily attributes and functions necessary to sustain life

Also: pass away, kick the bucket, cash in one's chips, buy the farm, give-up the ghost, drop dead, pop off, snuff it.