Sans is doing his best (or maybe his okay) to get the gelatin to wiggle like a moldsmal would. He's watching that rather than Papyrus.

"Sounds like a hassle." Maybe that's an understatement, no matter what specifically the hassle is. He does look at Papyrus again, though, searchingly--he doesn't think Papyrus would show up to a pool party if he was still drugged, but on the other hand, being drugged might inhibit that kind of logic.