I've heard that crocea, being rock boring clams by nature, should be placed on rock, and can actually be irritated or injured if placed on the sand bed. I can't say for sure, but that's what I heard.
My crocea is sitting in a depression in my rock work, about 3 inches from the water's surface, about 8-9 inches from my 150w MH, getting blasted with flow, and it's loving life.