It didn't though, Mick is still doing what he doing before and Steel developed and issue just to remove it instantly like it as never there.
A filler episode to me is one that does nothing to advance its characters or the story of the show.
In this episode we had:
1) Ray gets a way to rebuild his suit.
2) Steel learns that it's more than superpowers that make a superhero.
3) We learn that Mick still doesn't feel like a part of the team and could very easily slip back into his old ways, as evidenced by him becoming immediate friends with the episode's bad guy.
4) Futher to that we had Vixen and Mick's relationship advance.
5) We got setup for something interesting with Stein.
The last few bits there are going to be important for the show when it reintroduces Captain Cold, as Mick is still kind of in a place where he'd join back up with him. He needs to not be in that place and they're trying to build up loyalty with the team.
Sure, you could have done those things in any time period and episode, but enough personal advancement happened that I don't think it's fair to call this filler.