That thought crossed my mind when I saw this scene too.

My specific read on it was not necessarily that he's disappointed that it's Echo. I'm sure he's glad to see Echo. (Even if I feel zero chemistry in this pairing, it is what is written.) Just more like he's disappointed that Clarke isn't there with all of them. Seeing as Echo has both Madi and the Rover, he's gathered that there was some kind of confrontation with Clarke, and apparently that still led to Clarke staying behind, not joining them. Her leaving him in Polis really hurt him. I think it stings that she's still not coming to him the way he went to her every time she was in trouble. He's thinking that she just doesn't care anymore. About him or any of them.

But since you can be distraught and stressed about many things at once (trust me I know) I got the feeling he's also worrying about what they're about to do, the rift between him and his sister, the fate of the world, etc etc.