Isolate from others tapes to avoid spreading, try to control relative humidity in levels under 55%, open (if possible and without risk of loosing all the tape) the cases in order to air to flow freely and find someone. If as we see someone is not easy to find, search for cleaning products. If fungus doesn’t deteriorate yet the tape, you may be able to clean it without harming the data. If nobody/nothing appears, a basic and not too much effective solvent -but a safe one- can be isopropyl alcohol and can be applied in a not-too-much-wet cotton ball doing a single movement from left to right in the affected area, always with absorvent paper under.
If the fungus “stains” are more likely in the sides of the roll, a even less wet cotton may be applied with even less pression in order to avoir as much as possible to alcohol to rest between tapes turns.
Its not really nice to say this kind of things since fungus cleaning is a delicate thing but am gettin tired -in general- of not hearing a word from nodody when I ask for past experiences and solvents examples, and biological deterioration is progressive and must be stopped in time.