I think he just loses control. As for what kind of control specifically, that would depends on who he’s facing. I think the key implication here is that when he faces Seungjoo, he loses control over his libido, meaning at the very least he lusts over Seungjoo, and most probably is in love with him. In other cases, where he meets other exorcists or spirits for example, he’ll probably just lose control over his spiritual energy and strength. I don’t think he’ll go horny over others like he does in this chapter. That’s why it says that he doesn’t work during the period because he doesn’t want to be a burden to other exorcists, meaning he doesn’t worry he’ll be all over others as he does with Seungjoo.
so every full moon he gets horny