ID unknown hitchhiker (coral? pest?)

[c_scherer123]

I have no real way to describe this. It is about an inch round, oval shape. Last time I thoroughly inspected the live rock was about 3 weeks, that particular rock has been in my tank since April. It is in a very shady spot.
It seems to be uniformly attached to the rock (ie no clear base like aiptasia) and doesn't seem to react to touch, movement, or light change.
This particular rock has had problems with hair algae, though the other rocks are fine...
Can anyone ID it?
link to a larger/clearer picture: http://i445.photobucket.com/albums/q...r/unknown1.jpg