Big question. If Mold gets into Gum Arabic should you toss it>YES If mold gets into any of your paints or mediums throw them out
Toss your GA if it has mold . Mold can compromise your paint and there is no way to be sure you got it all out. If you want to avoid the problem buy dry(powdered) Gum Arabic
Gum Arabic is great for mixing with Watercolor paints and Pearl Ex. I use it for keeping my brushes in shape. After cleaning I dip them in GA and shape them. Let them dry. Next time you use the brush rinse out the GA.