What is the black seal on the inside of a vinyl replacement window made of--the seal taht is between the window glass and the vinyl? Does borax degrade it or make it expand? Am I ruining my windows?

I have been trying to get rid of mold that grows around my vinyl replacement windows. Mostly the mold grows at the bottom in the winter, when there is moisture collecting on the window. I have tried borax to get rid of the mold and let it sit for one day (1/2 c borax, 1/2 c vinegar, 1-3 c water). Now it looks like the rubber (or whatever the black seal is made) of is expanding or cracking. I can't see it well enough to tell exactly what is happening.