I'm pretty sure Nagisa has developed some kind of feelings for Mokuzu. And, Mokuzu likes Nagisa in a more than friendly way.
I personally think so too, but that's probably because I wear yuri goggles 24/7. The matter seems to really be very much open to discussion - Mokozu seems to have never had a friend before, so her clinginess could be put down to that, and Nagisa doesn't treat her as a friend until the last few chapters.
I loved the night scene with the two of them just talking by the sea, and my heart broke with the whole running away thing even though it's so cliched. If only they could have escaped and lived happily ever after. ToT
So if you put a slightly shoujo-ai tag on it despite the tiny amount of material which could be construed as their having feelings for each other, you might as well put a het tag on it because of Kanajima's crush on Mokuzu and Nagisa's crush on Kanajimi too. Even though I thoroughly support Mokuzu x Nagisa. It's probably significant that Nagisa protected Mokuzu from Kanajima and later even comforted her after Mokuzu beat Kanajima up, since normally you'd expect a person to fly to their crush's defence. I think I'm undermining myself a little here...
Hey, I think a one-sided love tag is also a cool thing.
I agree, I like that sort of thing too.