If the glass on your aquarium never gets dirty or grows algae (or this is only in for short periods) then you're in good shape, but this still seems like a very short term solution.
Personally I would catch the Butterfly and sell him....not hard (and you should get a nice price for one that's already feeding!), but most traditional methods are very hit or miss and typically require tons of time and a lot of patience unless you are willing to temporarily remove your rock. (Depending on your situation, not a huge deal if you plan it right.). A better alternative is a very small fly-fishing hook baited with his favorite food. Get the correct line for the hook too. File the barb off the hook if the idea of it bothers you, but wait til you see a (e.g.) #2 hook in person - it's already tiny, tiny, tiny! :) Very fast catch IME and 99% less stressful on him - and the rest of the tank - than an all-day chase or tank breakdown.
(If you can be patient and a little creative, even just a net - no chasing - can work. Can also require a lot of attention too though.)
Hope this helps! :)
-Matt