Trivia: Capitan Gonza's head looks just like the crone Yubaba's green bouncing head pets in director Hayao Miyazaki's 2001 film "Spirited Away."
Trivia: The dragonflies in the Deer God's clearing (when San brings the wounded Ashitaka to the God for judgement) are - except in size and ferocity - cameos from Yamazaki's and Studio Gihbli's first great hit, "Nausicaa of the Valley of the Wind" (1984). Many of the giant insects look quite alike to their tiny cousins featured here.
Trivia: After the disaster that resulted in the American heavily-edited and poorly dubbed release of NausicaƤ of the Valley of the Wind, (re-titled "Warriors of the Wind), and having heard that Miramax co-chairman Harvey Weinstein intended to cut Princess Mononoke to make it more marketable, one of Studio Ghibli's producers sent him an authentic katana with a simple message: "No cuts."
Answer: You can see part of the ooze flowing across Moro's neck, thus it gets severed.