Leaking freeze plug, and bare wire?

[s1ngletracker]

So I'm in the middle of a head replacement and there are a few things I've found that are concerning me and thought i'd get a second opinion.

First, looking at the back of the block, it seems like one of the freeze plugs are leaking. There is a fluid stain trail down the back of the block, coming from it. I should replace this, right? Should I replace all of them? How often do they leak? Best way to knock it out, is it hard?

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Second, There is a bare yellow wire on the harness that goes to the alternator plug. Any idea what this is for?

Pic:




Thanks for the help!