What do cucumbers (holothurid?) eat exactly?

[fredfish]

If if grows as a biofilm on sand grains, and some diatoms do, then yes, it will be eaten by holothurids. Macro & hair algaes, no. They are not an algae control critter. If you are looking for a generalized algae eater, look for a diedema species urchin (see the latest Reefkeeping online issue)

Fred.