I too noticed some confusion over this when I first introduced a co-worker to opengoo tasks.
Why not mimic the display in the Tasks tab, with the checkbox icon on the left with the "Complete" word? It gives an extra hint of what's going to happen.
To make it clearer and avoid confusion on whether it's a verb or adjective, perhaps substitute "Complete" with "Finish", "End", or "Close"?
On the other hand, well selected icons are known to convey such common actions effectively on their own. Why not simply remove the word and leave an icon in its place? The only drawback I can think of is making the click target shorter, which hampers usability (Fitt's Law).