Two years ago, when opening our inground vinyl pool, we noticed a depression on the shallow end of the pool. We called our local pool company who diagnosed this as a leak and patched it (glued about an 8 inch diameter piece of vinyl over the leak. Last year when we opened the pool, we noticed a rather large black stain around the patch, but we were not able to remove it through scrubbing. This year the stain has gotten bigger and is quite an eyesore. Is this some form of algae, some reaction to the glue or something else?