Midori's a lesbian in love with Tamako. However, so far Tamako has shown few signs of being a lesbian herself, so Midori is trying to keep her out of hetero relationships until she can (or so she dreams) turn Tamako into a homosexual. From that point of view I reckon her behavior did make perfect sense. What I was thinking during the ep was how funny it was, after a fashion, that Mochizou ended up hanging out with Midori a whole lot during this school trip, instead of the girl he really wanted to be with. And the exact same of course goes for Midori. Naturally it is a mystery how he's so weak about it at this point. You'd think he would be able to, at the very least, be in the company of his childhood friend, even if he's too shy to confess.