Hello ejking: Unless you're using a flor sherry yeast, there should not be "mold" on the surface of your must. Not knowing exactly which yeast you're using, it's hard to say whether you have a stuck fermentation or not as some wine yeasts display little indication that they're working. If you could describe the "mold" (color, texture, patterns, wet/dry etc...) it would help. Is it a dry, white, powdery looking substance? Also, rhubarb is rather acidic, and if your PH levels are too low it could stress, or even inhibit your fermentation. Did you treat with camden tabs, or pasturize? It's tough to attempt a diagnosis without more info. So, I need yeast type/designation; accurate description of the "mold"; type of must/fruit sanitation (if any); and your OG reading. With that we'll all be much more able to help. Regards, GF.