I have to go to the gym soon, so wil list a couple horror/human relationship manga. They aren't too similar to this one, buy are all decent. (I'll make s better list later.)
One quick horror-"romance" is Pupa. I still don't know how that manga was published in a shounen magazine. It is definitely rough, but nevertheless my baseline of what elements are required in good horror.
Hitoribocchi no Chikyuu Shinryaku is another, somewhat similar romance but has the distinct difference of everyone being human on the outside at least. Plus I wouldn't call it horror in a similar sense to the others.
There's also a romance between an alien butterfly girl and an abusive(?) boy, but I can't remember the name off the top of my head.
Lastly I'd recommend "the girl on the other side". Incredibly pretty manga, and filled to the brim with symbolism and "between the lines" story telling. It isn't really romance though, but more an adoptive family relation.
I'll come back with a better list later. These are just a couple general recommendatilns that I believe most people would enjoy, and Pupa that's just plain fucked up.