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.