Proteins that cannot form biofilm?
One option is to treat all proteins that are not explicitly known to form a biofilm as proteins that are incapable of doing so, but this will likely result in many false negatives.

You could take a look at one-class classification and positive-unlabeled (PU) learning. Both are techniques specifically designed for these kind of problems where only one of the two classes is known. (In this case: only biofilm forming proteins are known.)