I think it's perfectly legitimate to call this a bluff, despite the very real possibility that he's betting with the best hand. I could define a bluff as a bet that hopes everyone else will fold. Furthermore, someone with a hand like QJ is getting close to odds
to draw to overcard outs vs. a 5. They may not have the "best" hand, but they have a good enough hand to continue.
The way to consider it NOT a bluff is the hope that it gets called by a worse 5 who then checks it down after we check the turn and river. But that's a bit of a longshot.
So for all intents and purposes, this is a bluff. And a good time for it, too. Especially given that you want to bet these flops with weak hands so that you don't seem suspicious when you bet them with A-5 as well.