I'm going to assume its 5 inches long, 5 inches wide and 12 inches tall, is that correct? (thats 300 cubic inches, BTW
. only 25 inches square of surface area)
That seems a little small. Even if you made that 6 inches wide, by 5 by 12 tall it's still pretty small. You want a little more surface area with the sand to make it more efficient.
Caulerpa does need to be submerged, AFAIK
. It will also need light, obviously. So be sure to plan for placing some lighting over your fuge.