So here I am with an update. Took another 2 samples today to my friend, one from the aforementioned fish and another one that I found that was extracting the same mucous, and the microscope revealed lots of nasties... Not sure if I should be pleased or not. We sent the footage to the ichthyologist and are awaiting for her answer as we couldn't discern what the organisms were.
EDIT: Nothing potentially harmful identified in the samples, I'm as lost as ever...