There are any number of reasons for this challenge.
1- even though you can't "feel" anything, it may very well be mineral deposits. It may not be a steam shower, but the splash and small amount of spray that gets up there is probably not cleaned off and therefore accumulates.
2- degradation of the stone. It doesn't look like it, but because of the nature of stone in a shower, it might be.
3- an acid cleaner may have been used by the maid or someone else, not neutralized and simply left there to re-activate every time water hits it.
The cure? More likely than not, a wet hone starting at 220, 400, 800 and then 1,500. You could go higher, but that shouldn't be needed (we've gone the limit on black to 8,500). Finally, a good penetrating sealer like Crystal Magic Penetrating Sealer or Crystal Magic Color Enhancing Sealer.
Charge 'em up. We charge, depending on size and amount of walls and damage, $1,800 and up. It takes a long time to do, as the tech has his shoulders aching from holding the grinder flat. Remember to use something to prop up the tech (you?) so that you are holding the Makita no higher than should height, if possible. And, plenty of water. If you haven't got a pump system, build one so that you are constantly getting water thru as a lubricating agent.
