At first, I thought the same thing Timbal says-- that the player above the GM impersonator was just doing the trick where you add a bunch of spaces after whatever you type, and then write "[2. Trade] <GM>[Mike]: Hi" yourself (the chat window pushes everything after the spaces to the next line, making it look as though it's a legitimate chat message). Old trick, not really funny if overused (or if used as in the pic above), but it has its place.
But then I got confused-- Ohword says he could click the name, and the search actually came up with "0 players total," which shouldn't work, if it's just typed out. Also, his addon timestamps everything, and clearly the GM's line is timestamped, which I'd imagine it wouldn't be if someone was just joking around.
MVP Faizaniel suggests that it's just a player who was accidentally given a GM tag, but does that happen? I'm sure it's not a big deal-- even if this player did get a GM tag, he didn't actually have the power to ban anyone, so no harm done. But a strange occurrence just the same.