Is there a trick to separate an old photo moisture-fused to glass?

I agree with dpollitt, the parts of the photo stuck to the glass are most likely damaged beyond salvage. Although I've seen miracles come from someone who is well skilled in Photoshop!

That said, to separate the two from each other, I would first try using PEC-12 Photo Emulsion Cleaner.

You may also want to try and immersing the whole lot in a pan of distilled water. Using distilled water to prevent any damage from dissolved minerals. This may re-hydrate the stuck parts, and, as long as the print is not water soluble ink, just dry it quickly after and wipe it down with something like PEC-12.

If that doesn't work, as a last resort, I can only suggest using regular lighter fluid. I learned this trick from being a photo Grip for a catalog shop many years ago and would use lighter fluid to clean things like tape residue from all sorts of really expensive pieces of furniture and glass. I've also used it very successfully on other things like plastics and porcelain. All without any damage to anything.

If I had to guess, the slippery properties, and lighter fluid's ability to loosen things adhered to others come from it being petroleum based. But as mentioned, I would only do this as a last resort as I've never before applied it to a photograph.

I got a strange idea not sure whether can work out for you. Besides the fluid, or water, or oil which you can used to soak the photo so that you can separate the picture from glass, trying to limit the damage to lowest level. You may try to user camera to take a photo before you do all those job. If the photo you take is good enough, you can wash out this new photo.

Since I do photo restorations I frequently see the results of others efforts of trying to separate the photo from the glass.I personally do not try separating them. I do a high resolution flat bed scan through the glass and then rebuild the photo on my computer. This way my customer has both.

I had the same issue with my son's photograph which was only in the frame for a year. Anyhow, soaking did nothing. Nor did heat from a hair dryer. I ended up losing parts of my picture to the glass which I then had to scrape off from the glass with a razor blade. Needless to say, I will be scanning in this school photograph prior to placing it in the frame.

I used a very sharp scraper blade. It got the picture off the glass with minimal damage. There was some, however. Maybe if I had soaked it AND used the scraper blade, it would have worked even better.