The only thing clams are proven to eat is live phyto, like DTs. There are some phyto pastes that claim to be accepted by clams, but I don't know how much truth there is to those claims. It is true that croceas are found on rocks in nature; one of mine that's doing great has really burrowed into a hole in live rock I put it in. So moving it onto a rock might be a good idea.

Whether you have enough light would depend on your bulb and how deep the tank is. Lumenarcs are very efficient, so if you're running a high par bulb like a good 10k and you have a normal depth tank (not more than 24") I'd be very surprised if you don't have enough light.