Yes, MYOSES is a Scrabble word!

Letter values in Scrabble US: M 3, Y 4, O 1, S 1, E 1, S 1.

Is it a valid word?

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

an acute infectious disease occurring in epidemic form and featuring paroxysms of pain (usually in the chest)
reflex contraction of the sphincter muscle of the iris in response to a bright light (or certain drugs) causing the pupil to become smaller

Also: epidemic pleurodynia, epidemic myalgia, diaphragmatic pleurisy, bornholm disease, light reflex, pupillary reflex.