i just started using MTG (main tail groom) on my mare who needs to grow more mane hair. I've only put it on her once, because it says to only apply once a week. This week i was going to put it in again, nut i noticed that she had some blackish stuff running down her neck from a section of her mane. it's not liquid, basically like the texture of maybe pencil lead that you get on your finger. I rubbed it a little with my finger, and some of it came off, so it will probably wash off, but i was wondering if it might be from the sulfur in the MTG??? We do keep her outside if anyone is wondering. Should i stop using it? i don't want to , cause i just bought it, but is it safe??? If anyone has any experiences like this, or knows what it might be from, please tell me!!!!! thanks so much in advance!