I suspect that it may just be residual oil or cutting fluid in the mould .
It usually burns off but not always sometimes it seems to ooze out forever . The iron moulds are made from a very micro porus alloy , I can't remember the name now , but it will hold a lot of oil compared to what the surface looks like anyway . An alcohol bath might help but a couple more runs will most likely fix it also .

I had read for several years about the wonder of the brass mould and what great tools they were . I won't carry on but I was about 2 seconds from sending that little delight on it way .......then it just started working . Now I get the wonder of the brass mould but the start up has ensured that I won't buy a new one again .