What seems to be forming in reusable water bottles and water coolers is a biofilm. Since it is a complicated system, after maturation biofilm might be very hard to remove without scrubbing. There is a nice article about cleaning water coolers, which might give you some information.
If you look closer on the Google Scholar search results, you'll find that quite a bit of research has been conducted in the area of sanitation of water coolers and study of their flora. For example, consider the following paper from 1996: Analysis of the Virulence Characteristics of Bacteria Isolated from Bottled, Water Cooler, and Tap Water.
There is a very nice list (Table 1) of different bacteria isolated from water. But I would like to point your attention to following table from the paper:
Quote from methods section listing tested antibiotics:
natural and first generation antibiotics included:
penicillin, ampicillin, erythromycin, kanamycin,
streptomycin, tetracycline, chloramphenicol, and
sulphonamide. The synthetic and later generation
antibiotics included: gentamicin, cefoxitin,
cefoperazone, oxacillin, piperacillin, imipenem,
ciprofloxacin, and sulphonamideftrimethoprim
The green slime you observe, I think, is a combination of all those bacteria, plus some that were brought after the water was poured into the container. And it seems that all commercial sources that you didn't purify yourself, will have a variety of microorganisms growing. And around 5% of the total bacterial cell mass is resistant to common antibiotics.