My first mare would get pellet mush stuck in that exact same place.
We freaked called the vet and then we all had a good laugh afterwards.
Hopefully it's just something silly like my situation was, good luck!
Can you post another pic not standing under her... On a second look it could be further up than my horses "food stash"
My friends horse has that too. She has had it since she was born. The vet has came it looked at it and said it was nothing to ever worry about. Its not food but I forgot what he said it was. When her horses get their yearly shots he always just takes another look at it to much sure its not getting bigger or bothering her. Sorry I can't tell you what the vet said it was but at least it might give you a little piece of mind that it might not be anything bad