Glass eel. Tail pattern, shown in picture 5, fits that of A. bicolor pacifica, based on figure 4 of (https://www.researchgate.net/profile/Nico-Leander/publication/233945182_Species_Composition_and_Seasonal_Occurrence_of_Recruiting_Glass_Eels_Anguilla_spp_in_the_Hsiukuluan_River_Eastern_Taiwan/links/0fcfd512b26cc06685000000/Species-Composition-and-Seasonal-Occurrence-of-Recruiting-Glass-Eels-Anguilla-spp-in-the-Hsiukuluan-River-Eastern-Taiwan.pdf?origin=publication_detail&_tp=eyJjb250ZXh0Ijp7ImZpcnN0UGFnZSI6Il9kaXJlY3QiLCJwYWdlIjoicHVibGljYXRpb25Eb3dubG9hZCIsInByZXZpb3VzUGFnZSI6Il9kaXJlY3QifX0)

The dorsal fin is short and starts at where the black and red arrow is pointing at in photo 6,7, meaning this is a shortfin eel. No dorsal fin occurs in front of this point, as shown in photo 3, 8 and 9.
Can refer to photo 1 and 2 for dorsal fin length in proportion to body.

Same dorsal and anal fin starting point.

This is A. bicolor pacifica, if I am not wrong. A Hong Kong first record.

鮑氏雙足蜥 (香港雙足蜥) Dibamus bogadeki (Darevsky), 1992
有鱗目 Order: Squamata
雙足蜥下目 Infraoeder: Dibamia
雙足蜥科 Family: Dibamidae
雙足蜥屬 Genus: Dibamus

鮑氏雙足蜥( Dibamus bogadeki ) 研究
http://www.ngensis.com/DBL/DBL.htm

I watched with amazement as this young Striped Stream snake hoisted itself out of the water and crawled up the underside of the boulder, UPSIDE-DOWN, towards a patch of Hong Kong Cascade frog eggs. It fell several times in the process, either having failed to properly glue itself to the rock, or in the struggle to tear away the eggs. I'd seen an adult eating eggs on the same rock (previous photos), reaching down from the top to get to the prize.