Yea more than likely it is dead under or inside a rock,It must not have fully acclimated to your tank, with cleaners their hit or miss,Ive notice from experience,if you can keep them for a few weeks then your more likely to keep it longer,but if it just dissapears usually they died over night, Inotice both of mine sleep early and wake up early,thats how I know mine aren't dead. Usually if you wake up and Its not swiiming all over the place its a goner.