I would just go to WalMart and by myself a very stiff-bristled fingernail brush and use that.
Sometimes getting dried mud out of the frog area can be rough, but sand paper won't work between the frog and the sole anyway.
Sand discs are used by farriers who trim using the abrasive grinding method, but using sandpaper on a horse's hoof to get mud off is a new one on me and something I would never do.
My finger nail brushes and a quick buff afterward with a paper towel work just fine and there's no damage to the hoof Edited to Add:
If the mud is caked on really thick, I have used worn-out bot knives to dig away at the bulk of the mud, then finish with the brush and paper towel