In fiction, most of the time, fear is represented by livid face (White or even some blue). Teakin was blushing every time it happened. I'm not saying that you can't have redness if you are afraid but it's uncommon. And for the sonuvabitch, I'm pretty sure he will have a real reaction (of fear) when he will be in the boy's body.
That's a very valid point re depiction of fear in fiction! Given the timeline of the story (with Minjae being just a very recent substitute teacher in Taekin's school and having minimal interaction with the kid) and all the novel spoilers that point to Taekin having no time to pay heed to any kind of crush with everything happening to him, however, I'm more inclined to think that the artist intentionally made him red in the face to confuse the readers
Is the reason why Taekin's heartbeat accelerates when Minjae's near him because Minjae looks like that sonuvabitch? Maybe it's not a crush, but fear that makes him palpitate?