Can't tell without a pic, but if there's a nick in the side of the limb where the discoloration starts, it's an indication that the finish has been broken and moisture is getting in between the wood and the glass. If the affected area isn't large, sealing the dent with epoxy will likely stop it's progress. If there's some visible separation between the glass and wood on the edge of the limb, you might be able to drizzle some epoxy into it. Might be able to hold it open with a pin or razor blade edge while the epoxy soaks in. Then clamp it gently and sand and varnish after it dries.
If the spot is on the middle of the limb, not touching either edge, it indicates a glue starvation when the limbs were constructed. Not much you can do about that. Many bows with such defects last a long time before coming apart, but eventually the area does enlarge and the limb fails.
There's no way to replace the glass on the limbs. Could glue a thin layer of black glass over the clear glass, but that would change the weight substantially. Better to have a different set of limbs made.