actually a corcea would be fine under the T5 tek lights, the only thing is, good water quality, If you think that you cant keep corceas under T5s then how are some people about to ceep some amazingly colored (i mean so beautifly colored that your eyes hurt) under PC? There are some who can, some who cant, It depends on the clam, there like people, all individual, all there own!!!
I have no clams in my tank right now because i am about to upgrade, but ahve noticed that some of my SPS corals, can be in one place and start to loose color be moved over about 2 inches (same amount of flow and light as it did 2 inches over) but start being happy and growing?!?!?! very strange, they all now what they like and are picky thats what. lol. (sounds kinda crazy but i think its true)
Try to move the corcea up a little. Also how much flow is on it?