Cal notification won't go away.

Every now and then, I get the call notification popup on my Mac. In this case I wasn't even at my machine (and it was locked). Now I can't seem to dismiss it... https://preview.redd.it/hhezccchq87e1.png?width=698&format=png&auto=webp&s=176a469d205da40910d5b3983d2930735e2d26bc There's no 'X' that appears when I hover. The green & red buttons do nothing (and the call itself is long over). Is there an app I can kill?