Yes, ALS is a Scrabble word!

Letter values in Scrabble US: A 1, L 1, S 1.

Is it a valid word?

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

thickening of tissue in the motor tracts of the lateral columns and anterior horns of the spinal cord; results in progressive muscle atrophy that starts in the limbs
a silvery ductile metallic element found primarily in bauxite
a state in the southeastern United States on the Gulf of Mexico; one of the Confederate states during the American Civil War

Also: amyotrophic lateral sclerosis, lou gehrig's disease, atomic number 13, heart of dixie, camellia state.

Not playable in Scrabble US: ALABAMA.