Soak them in 99% isopropyl alcohol for a bit then rinse very well. Be careful with how long you you soak in the iso though cause I noticed it kind of shriveled one of my bags a bit like it started to have a negative affect on the mesh screen cause it soaked too long. Iso is some powerful solvent.
If the mold has been on there for a long time I would check out your mesh carefully to see if there are any brittle parts of it after you clean them well.
I wash them well between runs then hang dry in front of fans.