Headcanoning that was the moment Hoshiguma fell in love with Haruka.

I like to think Mio loved her throughout Ato but had to be compromised for the order. In the end, Mio will choose clan over her love.

Haruka on the other hand was too overworked and afraid to say what she might feel. Imagine loving someone yet not feeling worthy or yourself. Would she love Momoka Hanyuu or Haruka Shiino?

Okay my real headcanon is that Haruka and Mio definitely had a fling but couldn't be anything more. Mio knew that her loyalties lie with Kinsekikai and they might be a day where her love for this little goldfish will be tossed aside for the clan.