The gray-beige foamy scum is coagulated blood and juices. It forms when protein is boiled; the hot water causes the outside of the meat to seize up, which squishes the juices out of the cool center of the meat. You can watch this happen every time you grill steaks or burgers. To reduce the amount of foam-scum produced, or to eliminate its occurrence altogether, introduce room temperature meat to room temperature water in a pot over the lowest heat possible. Don't allow the water to break a simmer; in fact, the water shouldn't even move.

Scum isn't bad. It's just unsightly. You can skim it off or scoop it out during cooking by using a net designed to clean aquariums ($2.85 at Petco for a net so very finely woven that it almost feels like fabric), or you can allow it to cool, rinse off the chicken, and strain the stock through a sieve lined with coffee filters or through cheesecloth.

Both ginger and garlic, and sometimes onions, will react to cast iron and aluminum cookware, carbon-steel knives and other foods. It doesn't always happen, but there are garlic-ginger mixtures that are lime green or turquoise/teal.

I'm curious: are you Filipino? You know how they say there are 100,000 lasagna recipes because there are 100,000 nonnas (Italian grandmothers)? Pancit is the same way. Most pancit is made by sauteeing, not boiling, raw chunks of chicken (preferably skin-on) in oil with onions and garlic and sometimes ginger, adding sliced celery and cabbage and matchstick carrots, and then adding liquids, usually soy sauce and water, and finally the noodles. Sauteeing before boiling is a great way to slow down or halt the production of scum.

Could it be your pan? If it's unfinished aluminum, that might be it. What kind of water filter do you use? I remember when I would change my Brita filter, sometimes I'd see some charcoal in the water in the first use.

Whenever I make chicken stock or soup I always put the chicken into the water and let it boil for a while, maybe 15 minutes, before adding aromatics and vegetables as there is usually a greyish or beige foam. This lets me skim the foam off without it getting into the other ingredients. I'm not sure if the foam comes from the bones or the skin, but I think it's from the chicken in your case. It can definitely look a little greyish or dingy.

I guess we are all stumped. I see foam when I boil chicken but it is usually cream colored. Not sure about the grey. I don't think the ginger has anything to do with it.