Generally speaking opal colors are more likely to devit, but are you sure it's devit? Is it scum from fiber paper that wasn't burned out? Just asking questions, not assuming you don't know.
We need to know more about the firing to help you out.
It is only on the top side of the glass. It is kind of spotty scum and the fiber paper appears to have all burned off. I've not had any problem with that before.
Sometimes if you haven't pre-burned out your fiber paper the fumes can fall back onto the glass and cause scum. You can avoid this by pre-burning out your paper and venting your kiln during the firing. I just put a 1" kiln post to prop the lid open until 1000F.
This is only with regular fiber paper, not thin fire. You don't do a pre-burn out with thin fire.