There's another interesting thing going on in my new place. It was here from the start, actually. On our shower floor there's this dark pigmentation around the drain. The landlady had cleaned most of it off before we came, but there were slight traces of it that remained. She told us she had to use a sulfuric acid based cleanser and a very abrasive scrubby cloth to remove it. I think the problem is the texture of the plastic floor. It's got a texture so people won't slip, but I think all the micro hills and valleys are a very inviting home for the slimy creatures of the universe.