Considering the circumstances and the previous panel, isn't that supposed to be snot?
Honestly I think it was cum cause yuri was hard and in the next panel you can see him playing the drums without pants on so
knowing Yuri, he can just cum without hands and he had a boner when he was drumming wildly so it might be cum (￣∇￣")
IS cum, thats why title is "Bukkake festival" Bukkake is when someone eyaculates over you.
Hilarious, really
LMAO YACCHAN GOT CUM ON HIS FACE AND JUST FAINTED HAAHAHHA THAT'S SO FUCKING FUNNY (≧∀≦)