User blog comment:Omega natsu2/Fairy Tail's Ending/@comment-4449950-20140626081503/@comment-7418318-20140626205617

The Mystogan disguise is already busted, so that won't work. And as he said, he didn't think of it at the time of "Only enough time to pass by one another".

And as you said the Mystogan disguise was a big risk from the start (as was meeting with Fairy Tail for that matter), but he was willing to take the risk because it was about something that endangered a lot of lives (the entire Human race as later turned out to be). I doubt he would have taken the risk just to get a bit alone time with Erza.