The Tigertail Cucumber - Holothuria hilla - is an excellent sand sifter for tanks using a DSB (Deep Sand Bed). However, it will burrow under your live rock as it processes sand looking for food, so you may have to readjust your rockwork from time to time, or support the foundation with PVC to avoid its collapse. The main risk is when a cucumber gets too close to the intake of a powerhead or filter. Their death can wipe out a tank, so always know its location. Tigertail cucumbers will self-reproduce by tearing themselves virtually in half. My own personal experience resulted in at least seven cucumbers from the original single purchase. Typically Reef-safe.
The tentacles mop the sandbed looking for food and detritus.