I've successfully used Iodophor to suppress mold growth in hide glue.
Iodophor is a powerful disinfectant used in the dish rinse water in
restaurants; some of you have likely used it to disinfect brewing
equipment in amateur beer/wine making.

I put one or two drops into 1/4 cup of hydrated (liquid) glue; I have
no mold growth, and can detect no degradation of the strength of the
glue bonds.  The active ingredient, iodine, dissipates slowly with time
and temperature; after a week of temperature cycling in the glue pot,
I will add another drop of Iodophor to the pot.

I do take the precaution of covering the glue when it is not in use,
and I generally make a new batch every couple of weeks.

John D. Rhodes
Vancouver, WA