I was also having trouble with mold until I tried the suggestion,
possibly from someone on this list, to put the glue in a baby food
jar, keep it covered when not in use and store it in the refrigerator.

The jars fit easily in my Hold-Heet glue pot, and hold enough glue
for a lot of work.  I've had no problem with mold since going to this
method.

Bill Maxim - Maxim Piano Service
Columbia, SC