Hi Molly, I wouldn't recommend using anything that has blue/gree mold growing on it, although I'm a little confused by what you mean by "proof." Are you referring to the overnight starter that is meant to go in your bread recipe? Again, I wouldn't recommend using it, but the fact that it's growing mold in such a short time seems a bit worrisome. This might be a good opportunity to be sure that your starter is in tip top health by giving it a few days of twice a day feedings at room temperature. This will bolster your starter's resistance to mold and unfriendly bacteria and may also impact your overnight starter's resistance. 

I hope this helps! Let us know how it goes. 

Barb

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