The yellow rangefinder patch is typically part of the camera’s optical rangefinder assembly, and the colored surface is delicate enough that cleaning can remove or damage it. So the usual advice is: don’t try to clean it yourself.

A dim patch can sometimes be improved during a professional service, but the fix may range from basic disassembly and cleaning to much more involved optical work such as separating prisms, re-silvering, and re-cementing elements. That puts it well beyond typical DIY repair.

So yes, restoration may be possible, but the practical answer is to have it evaluated by a technician who specializes in vintage rangefinder CLA/restoration, ideally someone familiar with your exact model.