Unsightly Clam
The last place where you’d want to find something strange is in your food but unfortunately, that’s exactly what happened to this OP. They were just trying to enjoy a hotpot with clams when they spotted this stringy object growing out of one.
Thankfully, it was just a “byssus” or what you call a beard. They are filaments that clams, mussels, and other creatures of this family secrete to help them attach to a surface. We guess the chef missed cleaning this particular clam. Although it wasn’t anything dangerous, we’re pretty sure the OP didn’t finish the rest of their meal.