Some things MXTX doesn't spell out in TGCF
MXTX likes to leave certain things to our imagination, and several of them periodically lead readers to wonder what the missing info was. Two are especially intriguing:
What was Hua Cheng's password?
What did Hua Cheng's murals depict?
I think these can be grouped together because they both point to some aspect of Xie Lian that lives rent-free in Hua Cheng's mind: that is, Xie Lian's incredible sexual appeal.
The password: we'll never know, but we do know that it embarrasses Xie Lian hugely, and there's really only one thing that ever embarrasses Xie Lian. This is a guy who has no problem declaring publicly that he's impotent, and a rag-picker, and other things that would humiliate most people. But his own sexual allure freaks him out and triggers an extreme modesty reaction. Especially if it's coming from Hua Cheng.
So we can go to town about the password, and imagine whatever NSFW phrase about Xie Lian is most likely to make him want to hide under a rock ... and also turns him on with delight. (Hint: in SVSSS the phrase "heavenly pillar" turns up a few times.)
The murals: I don't think we need the murals spelled out - it's pretty easy to guess what they depict. Hua Cheng became a talented artist when he was still a kid - he made a drawing of Xie Lian to replace the missing temple idol when he was about 12 or 13. He has only one subject for his art: Xie Lian Xie Lian Xie Lian. And since he doesn't expect that anyone else will ever see his mural, he can indulge his fantasies.
And here's where we have to look at what religious belief means to Hua Cheng. Now, we know that Hua Cheng considers Xie Lian his god - he literally worships him. So any image or effigy of Xie Lian is, for HC, literally a religious image to be venerated. At the same time, HC's particular way of venerating XL is a mixture of religious fervor and sexual desire. (That combo is actually pretty common in religious imagining, from the sexuality in Hindu sacred texts to depictions of Jesus Christ as a nude and very sensuous man, etc.)
When, at the end, HC and XL get together as a couple, this deep belief doesn't leave Hua Cheng. Every time he rails his god, he's praying to him. And the more dominant he is, the more it delights XL, who plays up the drama of helplessness to the max. There is no one in the universe less helpless than Xie Lian, so HC's form of worship is pure pleasure for him - a freedom from all responsibility.
So painting the murals is a kind of devout religious experience for Hua Cheng, one that sustains him during the long period when he can't find Xie Lian and doesn't know if he'll ever find him again. Painting the images is a kind of prayer, like lighting incense. Only, of course, Hua Cheng probably doesn't simply bow to the images he has painted, but does other things too. They are, basically, the most elevated, intense kind of NSFW fan art. (Leave it to MXTX to really understand fandom and its obsessions.)
The murals are undoubtedly celebrations of Xie Lian's beauty, sexual allure, and - possibly - images of sex acts in the manner of some Hindu temple sculptures. They are there for Hua Cheng to pour his devotion into, and also for his relief during the centuries of his lonliness.
One of my favorite things in MXTX novels is her awareness of fandom behavior and the ways readers interact with her characters and stories in the web novel format. It's most obvious in SVSSS, but she inserts all sorts of in-jokes and even parodies directed at her fans in the other books too. Hua Cheng's NSFW art is exactly what fans do. HC isn't just XL's "most faithful believer," but also his biggest superfan.