That is how my monti cap grows as well. Grows very thick and down. Mine has grown in size as well, but it is from 1/8 inch to almost 1/4 inch thick. All my monti cap is like that. I gave a piece to a friend and it whorled in his tank.
That is an odd growth in that guy's tank. I think mine is an encrusting issue. When I first mounted it was attached at only a small contact area. IMO the frag is reaching for the rock since it knows it's there.