This omamori is a somewhat complicated historical and literary reference. The simplest explanation is that there was once a famous instrument (a biwa, a type of lute) named Seizan that was kept here at the temple. Unfortunately, the original has been lost, but we know of the instrument’s existence thanks to historical record. The more detailed story behind this omamori is found in those historical documents.
This biwa appears in the famous The Tale of the Heike. We are told that it was named “Seizan” (青山 “green mountain”) for the painting that decorated the front of the instrument. The painting depicted the moon rising above a lush green mountain in summer. Seizan was made in T’ang Dynasty China and was transmitted to Japan during the reign of Emperor Ninmyō (833-850).
Taira no Tsunemasa, a character in the The Tale of the Heike who is very talented at courtly arts including playing the biwa, brings Seizan to Ninna-ji Temple for safe keeping during the war. Tsunemasa had spent time at Ninna-ji as a child and was entrusted with Seizan for some time before he returns it to the temple. Sadly, Tsunemasa later dies in battle.
In the Noh play “Tsunemasa,” a monk at Ninna-ji holds a service to pray and mourn for Tsunemasa. Tsunemasa hears the prayers and appears at the temple as a ghost. He speaks of his longing to enjoy the courtly pastimes he once loved, and plays Seizan once more and performs a dance before disappearing back into the Asura Realm (an unpleasant realm where demigods are constantly at war).
You can read the entire script of the play (in Japanese and English!) as well as a more detailed synopsis here at The Noh.com.
Feel free to use the information in this post to explain your Seizan omamori to your friends, Japanese or otherwise they will be very impressed with your knowledge!