Sand sifters are fish or inverts that sift the sand looking for the critters that live there. This is their main source of nutrition as long as the infauna will last. After that, they will usually eat meaty foods introduced into the tank. Dead give aways for sand sifting fish is one that scoops up the sand w/ his mouth and filters it through his gills. It literally looks like he is trying to eat the sand.
There are also sand sifting star fish...they work their way slowly through the sand looking for the infauna and consuming it. Neither of these is something that would hitch hike in unnoticed (ordinarily).
There is a difference in sand sHifting and sand sIfting. A sand shifter is one that just moves through it and shifts the furniture around much like you would move a tree branch out of your way. These are great for DSB
as they aerate the upper levels of the sand as they move. Common Shifters include Nassarius snails, Queen Conchs, Fighting conchs...all great additions.
Sand sifters are ones that sift the sand looking for something to eat. Alot of the goby species are sifters. Just ask the LFS
if he is a sand sifter, or better yet...ask here.