I posed this question to the list late last May and still do not have a
definitive answer on the subject, so here it is again.  Comments from the
list are welcome:-

The following link is microscope picture of some mold or mildew growing
on the covers of some 7" reel tape boxes issued by Capitol Records for
the Hi-Q library.

This is what the box looks like... the mold is brownish grey and dust-like...
on this box, it is most obvious within the rectangle between the words
"underscore" and the timings:-

I suspect the mold is feeding on the varnish that the printer used for a
gloss surface on the box labels.

Suggestions for cleaning this off have included (given in order of most
likelihood to succeed without damage to the artifact)...

1.  Vacuum first with a HEPA vac. Mold is very fragile, not tenacious.
After vacuuming, wipe with a 70% solution of ethanol and distilled water.
The 70% solution allows for enough water to hydrate the mold spores
so it will accept the ethanol.
Saturate absorbent cloths cut in small pieces.  Wipe off the covers, spine,
and inside around the edges of the wraparound of the cover, and down the
inside of the spine with a Q-tip if necessary.

2. Wiping with a cloth moistened with white household vinegar (5% acetic
acid) is said to remove most of it and supposedly kill any remaining spores,
although it is suggested by others that the vinegar probably won't kill the
mold spores.

3. Another suggestion is 10% chlorine bleach (Javex) in water, but it might
bleach the paper, which preferably is to be avoided.

Any comments or suggestions on this subject would be appreciated.

... Graham Newton

Audio Restoration by Graham Newton,
World class professional services applied to tape or phonograph records for
consumers and re-releases, featuring CEDAR's new CAMBRIDGE process De-Clickle!