looks like copper oxide. best thing is to use battery acid to clean it out of the carpet. Its not the easiest chemical to source but I found a way. Take two new 9 volt batteries and set them on top of each other with the positive and negative ends touching the negative and positive ends of the other battery. You'll feel them start to warm up which is how you know its working. Battery acid will gently pop out, at which point you smear it over the copper oxide stain on your carpet and leave it to soak, preferably over night. In the morning pour some bleach onto it to neutralize the acidity and it will wipe right off like magic.
