Whew... I saw the title of the post and my first thought was, a dog that is growing moss is having some serious health issues!!
Pokerguy! Cayenne pepper might work, but wouldn't it irritate the dog's tender skin along with removing its moss? <grin>
There once was a group of people who lived high above tree line in an area where not much grew other than moss. Fortunately for them there was a healthy market for moss down in the low lands. Each year these villagers would head up into the moss fields and gather up the moss by cutting it into long rectangles and rolling it up. Then they would transport it down to the lowlands and sell it to the eager buyers there. It was a pretty specific way to make a living, but apparently lucrative enough to provide them with the money they needed to get by the rest of the year. That was good since there was darn little else they could do up there to bring in money.
Then, someone introduced a wonderful plant to them that they found made them higher than a kite when they consumed it. In short order the entire village was wasted on this stuff. Happy, but oblivious. So much so that the moss harvest season came and went not so much as one villager went out for the harvest. As a result they had no income that year, and with no other source of funds coming in they were all forced to leave their idyllic high mountain village and head into the cities to work in factories. It was sad.
The moral of the story of course, is that stoned rollers gather no moss.
This is no comment on the legalization of marijuana by the way. Just a silly pun story I heard long ago and couldn't resist inflicting on you all!