Have you ever seen the inside of a drinking fountain? They accumulate quite a bit of “greenish slime” too. Or even the inside of the pipe feeding your kitchen sink? They all accumulate deposits, but we drink it and it doesn’t hurt us.
I kind of laugh at the people who won’t drink the city water, but pay high prices to fill bottles of water in the grocery store – where do they think that water comes from? It’s piped directly from the city water system, just run thru filters at the grocery store. And how often do you think those filters are cleaned by the stockboys at the store? I’ve seen them doing that late at night, with the machine open – there’s a whole lot of ‘slime’ deposits in those machines, too. This seems to be common among immigrants – possibly they are not used to trustworthy municipal water systems.