Methylobacterium residue doesn't look like slime (I used the word "scum" which wasn't the best word either). It gives a slight salmon-coloured tinge to the bottom of a shower curtain and sometimes a faint residue can be seen in hard to reach areas. It washes out easily and can be cleaned with the usual tub cleansers.

Serratia m. is a serious drug-resistant bacteria found in some hospitals (that is also pink) but that isn't what I was referring to.