What is this aquatic organism in Fiji?
It is probably Holothuria leucospilota

Holothuria leucospilota is a medium-sized sea cucumber reaching a length of up to 40 centimetres (16 in) when relaxed but it can stretch to about a metre (yard) when extended.

Holothuria leucospilota is found in shallow water along the east coast of Africa and in much of the Indo-Pacific region. It is a common species on the north east coast of Australia where it is found on reefs and rocky coasts, often partly concealed under a boulder.

References
https://en.wikipedia.org/wiki/Holothuria_leucospilota