I found them under reefs in Fiji. The hand-like structures moved slowly. I was not sure if it was an animal or plant. I was not even sure if they were several individuals or it was a whole. The part that was not hidden by the stone was about 50cm length. I did not touch them, since I was not sure if it was dangerous.
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.