Worth More than a Million - Yuri! On Ice fanfic

Viktor found a man playing violin in a room, where the moonlight shines upon him one night. It was a sad sonata. The next night, he found him again. The man's name was Yuuri. Viktor found out he only appears during the night where the moon is out.

Find out the mystery why Yuuri only appears at night, and what is his connection with Viktor.