The grey skin is Agyria, there is apparently a cure selenium & vitamine E slowly remove it from your system. Agyria and the scare stories around it have been discussed frequently on this board, I can't remember anyone here whose suffered from it & several people have been taking it for years. I've been taking CS (home made) almost every day for about a year now & have experienced no problems. Basically Argyria is really only a problem if you use silver salts, CS made with protein fixers or very large quantities of poorly made CS (made with constant voltage rather than constant current generator). If it is made with distilled water & a constant current CS generator you should be fine.
I'm quite pleasantly surprised that a Dr is prescribing CS, this may be a first. If you are in any doubt about the type of CS he's prescribing post details of the product up here & someone may well know about it.