If you are right, then that would mean: I am wrong and it is not the 'Lagoon monster' or the credits are wrong and the 'Lagoon monster' is not played by Yuen Kin Leung, but by Kenji Tanigaki.
I think I am wrong and misunderstood the subtitling.
The 'Lagoon monster' = the weird boy with the videocam and not (what I thought) the one he taped as the one who shot the undercoveragent.