I just mistakenly sent a transport mission from a colony then abandoned it . I want it to be a deploy mission to get the res out before i abandoned it, and Now on the overview screen its got a return.. any ideas as to whats gunna happen with my fleett?

You can still send attack missions to the destroyed planet by entering coordinates through the fleet screen.

That may be only until they do universe cleanup and delete all the destroyed planets or before that, I am not sure.So don't wait to do it.