is a black knoby Stichopus chloronotus
. They do best is sandy substrate tanks and should only be housed in tank that are mature and have enough surfcae substrate to allow for constant grazing. If the tank is too young, the cuke
can easily starve and food supplimentation is near impossible. They are indeed toxic although less so than most others. If in a smaller tank, they could easily nuke the water quality causing a fair amount of animal loss.
The second looks like a type of rock boring urchin Echinometra mathaei
and will most likely make short work of your coralline over time...
Moved to Saltwater- Identification