Aphids suck, not chew, and they would be present and visible in the daytime. A typical guess for sedum telephium would be slugs or snails, and you may need to go out after dark and check for yourself to know for sure. How are your temps after dinner ;)

Slugs are active here now, I don't know about your zone. Is the damage recent or just recently noticed....

in january in z6????? ... or was this back in summer ...

a pic will get you far.. with real help ...

ken

I wouldn't worry about it. The above-ground parts can't survive winter in NJ anyway, surprised there's still anything there now.