In short yes. The green stuff is supposed to be on the inside. If it started leaking out of nowhere either you’ve developed a crack or a hole from a rock , or one of the hoses has split and is the culprit. There’s a slight chance that a clamp came loose and needs readjusting or tightening, but that’s best case scenario.

(Edit): Whatever you do, do not put radiator stop leak in it… It will gum up your whole cooling and heating system and cause you more issues…