Hello Dylan.
Thank you for sharing this interesting experience.
I must admit that I haven't a clue as to what it might be, so I'm pleased that Alex has started us off with a sensible suggestion.
Whenever I have something like this that I find puzzling, I tend to approach it in a methodical way so that I can satisfy myself that I have done all I can to try to find answers.
What I would do in this scenario is not simply keep wiping the substance off, but put a folded tissue in place to try to identify more precisely from where it is emerging. I would also retain the 'sample' and ask my doctor if it could be analysed so that the substance could be properly identified. This would also give us a clue as to what it is and what might be causing it to 'leak'. Gathering such 'evidence', usually helps to pinpoint what might be amiss. I tend to adopt this approach with identifying where bleeding is taking place somewhere around my stoma. Sometimes the spots where it leaks are so small as to be invisible to the eye at the distance we are from the source. In the case of a Barbie bum, it would be almost impossible to see without a mirror or a camera of some sort. Even then I suspect it would need considerable magnification to be sure of what is causing the problem.
I recall many years ago, when I was having problems with hair follicles, my GP did not seem very interested. I eventually found a powerful microscope in the engineering department at the University that I was attending and took a photograph of the offending area and the hair that was causing it. This 'evidence' was enough to peek the interest of my GP and get him to prescribe some ointment for folliculitis - which has been successful ever since.
Sometimes, when things are not blindingly obvious to doctors, they adopt an attitude of 'there seems to be nothing wrong', rather than one of 'well, let's see if we can find out'.
Whilst working in the field of Mental Health, I came across several cases where ailments had been diagnosed as 'psychological' simply because the doctor concerned had not shown due diligence in their initial assessments. This is particularly relevant when it comes to digestive disorders, where anti-psychotics have been prescribed instead of antibiotics. It's a dead giveaway, as to what is perceived by the prescriber.
It sounds as if you are being told only what they are 'seeing', rather than trying to find out what exactly is happening. Perhaps it is time for a slightly different approach and, if necessary, with a different set of doctors.
Just a thought!
Best wishes
Bill
