I have a few different fluorescent microscopes, the two that use Mercury lamps are older than 7 years. I notice on those that the thin film filters in the filter cubes (green, red, blue) have accumulated this haziness. Both the excitation and barrier filters seem to have this, more so on the ex filter and it's not present on the dichroic.
I've always referred to this as the filter being "burned" and my rational is that it's from the heat & intensity & low wavelengths from the Mercury lamp. But in all honesty I don't know what it is or if it's removable and if so how? I would like to see if removing it improves my image clarity.
I know a picture would help here and I'm having trouble showing it but will keep attempting and add it to an edit for this question. However I have seen this on many microscopes if you have one with a merc arc lamp and it's getting up there in age take a look at the filters on it. Let me know if you see this too.