Yes, FLUX is a Scrabble word!

Letter values in Scrabble US: F 4, L 1, U 1, X 8.

Is it a valid word?

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

the rate of flow of energy or particles across a given surface
a flow or discharge
a substance added to molten metals to bond with impurities that can then be readily removed
excessive discharge of liquid from a cavity or organ (as in watery diarrhea)
a state of uncertainty about what should be done (usually following some important event) preceding the establishment of a new direction of action
the lines of force surrounding a permanent magnet or a moving charged particle
(physics) the number of changes in energy flow across a given surface per unit area
in constant change

Also: state of flux, magnetic field, magnetic flux, flux density.