I agree that Heejae has good intentions, but not the rest of your comment. I think Garam has been pretty upfront with his feelings with Heejae, but Heejae dismisses them with words to the effect of "You said you would be happy even if I was your only listener", or "If you have the time to worry, you should focus on improving your sound", both of which do nothing to assuage Garam's fears. Heejae is just too emotionally stunted to understand how to effectively comfort Garam. He tries to, through initiating sex or using indirect gestures, but that's not the same as direct communication. What's more, Heejae's attitude is rubbing off Garam, particularly because he doesn't have any social connections outside Heejae, the staff, and Walter.
That's a fair point, but I still think the root of the problem is their communication. I actually agree with you that there's almost no proper, direct conversation between them. Garam keeps saying, stop making decisions for me, but he never clearly explains what he wants to do instead. Meanwhile, Heejae constantly brushes aside Garam's feelings and basically keeps saying, trust me, I'm paving the way for you. You just don't see it yet, be grateful. They're both making assumptions instead of really listening to each other, and that's what keeps pushing them further apart, in my opinion.

I don't think either of them is acting out of bad intentions. Heejae genuinely believes he's helping Garam get more recognition and grow as a musician. On the other hand, Garam starts feeling like he's not good enough—that he's not as talented as Heejae's other students and only got to perform because of Heejae's help. If they had just talked honestly about it, they could've worked things out. But Heejae expects Garam to simply trust and follow his lead, while Garam keeps bottling up his feelings of inadequacy until they come out as anger. Proper communication is really the key here, and sadly, that's exactly what they're missing.