M.P.
This sounds like an oily stain. It could be from your detergent. I'm not familiar with Charlies Soap but if it has a fat base which is what a soap has I suggest it's the detergent. Fabric softeners can also leave oily spots.
I don't think there is any way for the machine to leak oil into the drum.
I googled Charlies Soap and found it's made of coconut and mineral oils. http://www.charliesoap.com/faq.html#one
I'd try using a regular detergent and see if that helps.
To get rid of the oil spots rub with Dawn dishwashing detergent and rewash. I've also been able to get rid of them with a prewash stain removal spray.