moist cellar smell
is unually what you already have guessed: A musty odor usually points the way to mildew or some fungus (aspergilla nigra usually, if not others).
The bad news: Different from what the average Joe believes, mildew/yeasts/fungus will ALSO inhabit synthetic materials, They love all soft and pliable stuff, especially rubber (real rubber the most, a bit less the neoprene stuff). Vinyl is ok for them for a quick nibble and some nesting in, but it is not their favorite.
The good (and bad news): Some chemicals (harmless ones as well as nasty ones) can kill the beasts.
What has to be kept in mind:
Fungi will dig small tunnels in every material they get hold of in order to spread their small threads (the "roots" if you will - once there is a fungus IN the material, the latter is usually rotten or destroyed for good). From there they rise to the surface, then showing us their blackish spotted or their greenish and hairy "blossoms" to spread their spores again... (and so on) - the plant comparison is wrong, fungi are not real plants per se.
How cleaning can work:
In all cases, try to wash off any organic material that does not belong there. As long as mildew has only settled ON the stuff, you may be lucky. Once settled IN (and the perforation has already begun) then the item you try to clean is a case for the incinerator.
After some serious washing try to kill the remaining organisms, there are basically 2 ways for this (A or B, see below):
A- an overdose of oxygen or activated oxygen (so chlorine or ozone)
Commercial anti-fungus sprays contain sodium hypochlorite, that is bascially caustic chlorox crystals like in Drano or Amway oven cleaner and all other DIY fungus wonder potions)
Milder but still enough chlorine = Clorox and all its derivates (also mildly caustic, use gloves when working with all of the above)
"activated" (read: aggressive) oxygen = ozone O3. Expose the item to be cleaned in ozonated air (ozone generators can be had at car shops for a small renting fee per day)
Bad about all oxygen/chlorine action: Caustic. May bleach or deteriorate your material.
B -Killing off the organic remainders with suitable discinfectants (here: mostly alcohol and other disinfectants).
Use lamp spirit, isopropanol, any alcoholic disinfectant or the ones containing anything "benz.." or "alcyl..."
Good: Non caustic. Will not smell, will not etch colors out. Bad: Will not kill deep down into the "tunnels" in the material, so the fungus might come back eventually.
Other than the two methods above:
The C method (all sour/acid stuff like concentrated vinegar, which the fungi do not like)
and D method (scent masking, Febreze, Airwick) will only be temporarily.
A or B is the way to kill them off (IF you are lucky and your item is NOT YET perforated).
Good luck.
