Yes, mus is a Scrabble word!
...and is worth 5 points in Scrabble, and 7 points in Words with Friends
mus
Noun
1. Type genus of the Muridae: common house mice; the tips of the upper incisors have a square notch
2. The 12th letter of the Greek alphabet
What Scrabble words can I make with the letters in mus?
3 Letter Words
2 Letter Words