Thrush is black-grayish tarry looking stuff. light brown-white-ish-colored crumbly stuff
That sounds more like the sole is flaking out. It looks kind of chalky. It is normal for the sole to flake and look chalking now and then. That is how the old stuff sheds off and reveals the fresh stuff underneath.
While thrush can get into the sole, the first place it attacks is the frog and the grooves around the frog. Is the frog blackish/oozy, nasty looking? If so, that is thrush.
I will often treat my guys for thrush just at the start of something smelly/black, before it ever gets bad. I use a generic Coppertox type product. Some people feel that is rather strong and can damage healthy tissue, but I personally haven't had any negative issues with using it. And you get more for your money that some of the other thrush products. The one made by Absorbine is pretty good too. It is a clear liquid that kind of smells like onions.
The vet is your call. If it's not a horrible case of thrush (and she's sound) then I would just try treating it with an over-the-counter product. Picking the feet daily to expose them to air is one of the best things you can do for thrush. Even if they just go out and step in the mud again. At least the area is exposed to air which can kill/discourage the bacteria from getting worse.