I too am struggling a bit with my small pot of Serpens . It gets bright indirect light, temps down to low 60’s at night. I let It dry out quite a bit and i do feel like that is where I am going wrong. One of my 3 stems is shriveling likely to the point where I will lose the stem (no yellowing though). I have also read it prefers more moist conditions, I am just nervous to try that while we are still in winter, I have already lost a few of my hoyas to rot. My apologies for not really adding much to your post, just sharing my experience, just a guess but I feel like mine is drying too much. Good luck with yours! Katie