Mika was not afraid of ghost because of how they looked or they may be able to kill him rather because he thinks they are gross and that's reason why he feared them, like a girl's reaction upon encountering cockroach or spider or moth or caterpillar, and it's only an example.
This example does not, really really really not mean, he is like a girl, like his elder brother used to refer him, as a cute middle school girl.
Leave the reason's of disliking and all, all in all, he doesn't want this practical to get successed.
Mika started to think hard, should he stop in mid to wait Usami to make some distance between them and run away the very moment of finding an opening, no, then Usami will catch him and punish him to stay in haunted house for the whole night or maybe even the next few days.
