A water spot appeared on my basement ceiling, so I cut a hole to explore what was going on. I found the following:

my pipes are leaky

Something is going on with the pipe, a growth of some sorts. I'm assuming it isn't alive so it must be rust? Was the pipe just never properly fitted together?

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    Do you have hard water, or water with lots of minerals? It looks like there was a small leak, and you're basically growing stalactites. – Tester101 Sep 12 '14 at 20:45
  • No I do not believe I have hard water - I.e. no water softener and it tastes great to me, and my dishes do not get all nasty. – dpollitt Sep 12 '14 at 20:59
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    This could be a case of the original solder job being done with an acid core solder that was never neutralized and properly cleaned up. The acid could be eating a hole through the copper pipe which in turn lets water come out and work at further corroding the pipe joint. – Michael Karas Sep 13 '14 at 2:08
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    I agree, this looks like typical acid flux syndrome. cut out that tee and solder in a new fitting before you spring a serious leak. – Jimmy Fix-it Sep 13 '14 at 2:56

It looks like the tee in that spot is in need of repair. It likely wasn't sweated (soldered) properly.

If you don't fix it, it will likely continue to grow. So in a sense, it is alive...

EDIT: I was wrong in an earlier version of this answer where I said that copper oxide is blue: it's actually black. The blue/green that you see may be due to minerals in the water, like sulfates, which react with the copper. Others mentioned in the comments to your question that it may be due to the acid flux that was originally used when sweating the joint. A flux like that has HCl and have salts that can react with copper to form copper chloride, which is green.

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    More importantly, if you don't fix it the copper will continue to be eroded away and the leak will get progressively worse. – keshlam Sep 12 '14 at 21:21

Could also be that this is the cold pipe and is uninsulated. In the summer, the pipe will 'sweat' and get wet; the adjacent galvanized pipes and/or hangars may be dissolving out and precipitating on your pipe. But I'd change out the tee first.

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