What World Of Warcraft Add-on is this?

Considering it's showing the health of your target, and what appears to be a broken reference starting with "TInterface", I suspect it might be one of the extra X-Perl sub-modules. Try disabling only X-Perl and restarting, to see if it still shows up. Similarly, if it's still there, you can always disable one AddOn at a time, restart the game, and see if it shows up.