Really, really wired "squatter" found on SPS

[Cujo#31]

Today, while looking endlessly at a frag of mine that has taken its own sweet time to show even a hint of color, and limited encrusting after 5 months. Any how, staring at this piece, almost trying to will it to color up I see a polyp acting as it shouldn't. Extending and retracting very quickly (for SPS polyp standards anyhow) so I start looking very closely.
First thing I notice is the polyp isn't shaped like any polyp on the piece. It has a shape more like a barnacle. It was housed in a structure the same size as every corallite around it, same color. Only real difference was no "nest" for a polyp like you see in most corallites.
I tried a hundred times to get a pic but 2 problems, polyp was too fast in and out, and it was shaped so much like a corallite even I couldn't pick it up in a picture.
Tried searching endlessly, asked a few guys smarter than me and still no idea. I finally pulled the frag and tried to pick it off. Came off effortlessly, and left no damage to the skeleton behind. Not even a white spot where it anchored and there would have been an absence of zooxanthellae. Super, super weird. Any ideas? An NO....I don't have any pictures, and when it popped off it kind of crimbled and did not resemble the original structure at all.