This glass is held in a lead framework (aka leaded light). You can see how the lead has oxidised into lead oxide, a white powder which isn't nice to look at. Cleaning them is tricky, and a specilaist job. Water used to wash them will hold lead oxide so it would be poisonous, so must be disposed of carefully. Camefix is made of plastic, will not oxidise; and if the camefix stained glass panel is inside a sealed unit, both outer surfaces can be cleaned easily with your favourite cleaning product.