Yes, muck is a Scrabble word!
...and is worth 12 points in Scrabble, and 15 points in Words with Friends
muck
Verb
1. Remove muck, clear away muck, as in a mine
2. Spread manure, as for fertilization
3. Soil with mud, muck, or mire
Noun
1. Any thick, viscous matter
2. Fecal matter of animals
What Scrabble words can I make with the letters in muck?
3 Letter Words
2 Letter Words