I've been sensing this for quite a while, but it really seems like Elma thinks the world of Tooru and it might be a little more than mere friendship to her. There are just too many hurdles in her way though, from being a harmony dragon to Kobayashi-san's entire existence. I think Elma has already resigned to just being friends, hence the pained expression.
It's not like I haven't considered it before... but Kobayashi doesn't live forever.... just maybe, even if it takes centuries... Elma might become Tooru's rebound. Tooru said she would love Kobayashi for the rest of her life even long after Kobayshi is no more... but that doesn't mean she doesn't have a place for someone else in her heart eventually. Tooru and Elma have very good chemistry(tsundere Tooru is a treat) and are euqal in many ways, something she doesn't have with Kobayashi.
It's a delicate issue and I honestly thought Cool-sensei would skirt around it more, but this chapter was surprisingly direct about it. Even Tooru noticed it. Once again I have to concede that I am stupid for thinking she is dense. Tooru has proven time ad time again that below her airheaded attitude she is very sharp. Her embarrassed reaction here tells me exactly that she thought the exact same thing as all of us.
Looking forward to exploring this more, but I know that Tooru x Kobayashi is the end goal. I hope that we don't have a last minute swerve...