Maybe take it down and show to the algae you expect it to eat. I tried one last year when I had about one half a square foot of bryopsis hair algae throughout my tank. Yet it still shrunk and disappeared after one month because it seemingly couldnt locate the patches. I wont buy one ever again but good luck with yours.
There is some information on this page and in the related messages near the bottom.
http://www.seaslugforum.net/factsheet/elyscris