Weird black circle on inside of glass?

[level16gazebo]

So, like many of you, I have painted my back glass black. Like many of you, I have allowed coralline to grow over that, because scraping the back sucks and I don't really care. I purchased a tuxedo urchin the other day, and it's stripping the coralline off the glass. Fine, whatever, don't particularly care. Would rather you clean my rocks, but it's not a big deal. However, I noticed one spot in particular, a circle of about three inches, was completely stripped. Weird. Go to look at it... and there's some thin black film on the INSIDE of the glass. If you move your head from side to side, you can clearly see that you're not seeing the paint. I do have numerous black sponges in my tank, but this seems to have appeared overnight, or at least in the last couple of days... Anyone seen anything like this? I would post a picture, but there's not much to see. It's a black circle that's not paint, when I expected to be seeing paint.

 

[ZoWhat]

Oh yeah....?

... in the reflection of my tank glass at night