Many years ago I saw that exact thing after only a couple of days of running a brand new UV. Frosted glass, only where the UV light would have been, not on the edges. Very uniform, very white. If I remember right, I expected it to be calcium and poured some acid on it expecting it to bubble, but it didn't (if I remember right). I think I just cleaned it with just water. It came off easy in flakes, like it wasn't that well attached.
I was seriously bummed, there's no way the UV was going to work because I assumed the glass would frost over again in a few days...why wouldn't it? Same pond water, same UV. I checked it regularly but the frost never appeared again even over the next couple of years.
Would be great if you could confirm this does or doesn't react with acid. Vinegar should be good enough. Been driving me crazy for years.
In all the years since I've only seen one other post from someone. I found a plausible scientific explanation once, but then pretty much completely forgot it. I think maybe it had to due with the high temp of the bulb causing something to precipitate. These bulbs can run extremely hot internally. But that didn't explain why it didn't return. My only guess was there was something about the surface of the glass that allowed precipitation or not. Same reason Mentos make soda foam. When I cleaned the glass the texture was changed and it never happened again, although I wish it had so I could have done some experiments.
Given your picture, the other post and my experience the thickness of the film seems to always be the same. A bit less than the thickness of paper. Very uniform, a bit rough but no bumps.
Any ways, I would check it again in a few days and then maybe once a month. I can appear quickly.
