That is from true cavitation... actually inertial cavitation in your case. A low pressure area is being formed which creates a space in the water. As the vacuum space returns to normal pressure, the space collapses and a shock wave is created which is damaging to the metal. This is where your pitting is coming from.
Something is creating this, I once had this happen half way up the blades on my ski boat prop. The culprit was a slightly bent skeg. I straightened the skeg and the 'pitting' went away. Before I straightened it, I could spray paint the prop and run it one time and the paint would be gone in the pitted area. After straightening the skeg, the paint stayed on.
You have something that is creating the vacuum around your prop. Either a deformity on your outboards foot or on the prop. If it is only on one blade, something is wrong with that blade on your prop. If it is on all 3, the problem is on your foot or gear housing.
:tu: