It is actually probably the paint on the silvering starting to bubble off, think of "schniederitus". If the silvering itself has been affected the viewfinder image will appear hazy. I have a couple of prisms where this has happened and they still work fine. On the other hand I have several prisms where the silvering has degraded without any bubbling of the black backing paint. If it works it works, if it doesn't it doesn't.
If you can see through the silvering to the paint bubbling off, then the silvering is bad, too.