I remember the first time I read this chapter about 2 years ago. I was still very naive. I thought the secret that he's talking about was his fear of Aoki gonna end up dead like Suzuki. The idea of Maki having feeling for Aoki didn't cross my mind at all. Only the second time I read this that I finally realized lol
As this manga isn't tagged as BL, I'm actually never sure what kind of feeling Maki has for Aoki. But I'm sure he wants Aoki for himself. Thus he's jealous of Miyoshi.