All members of the Class: Holothuroidea are toxic as they contain a poisoness substance called Holothurin, hence their name. Many genera can poison a tank by evisceration (expelling their guts), caused by bad aquarium conditions, harassment, overcrowding, or even getting stung by corals or anemones, etc. Others expel Culvierien Tubules containing Holthurin out of their anus when being attacked or harassed causing death to their attackers.
Others can poison a tank by spawning.
Good skimming, activated carbon on hand and saltwater for water changes is a must if a cucumber decides to nuke it's environment.
Of course in many cases, in death they cause no harm to the tank what so ever.
If you keep one be vigilant and know that you may have to act fast to save your other tank inhabitants.
Regards,
David Mohr