For expert on Japanese Murrelet, I recommend to refer to Mr Koji Ono 小野宏治 (ono@seabird.go.jp). You may also visit his webpage www.seabird.go.jp. He was really helpful in telling us how to go to Kadogawa to see the murrelet in 2001, despite we were strangers.
I am really excited to notice this sighting in Hong Kong. Actually for Japanese Murrelet, the breeding season is late Feb to May. The largest breeding colony is in Kadogawa, Miyazaki Prefecture in Kyushu (~3000 birds or 1/2 world population). Even for Taiwan, there seems just only a few sightings. This bird, if proven to be a Japanese Murrelet, is likely strayed due to injury or disease.