I did alot of research before I bought my clam and read the advice of people who import and even raise these clams. All of them advise feeding small clams, and as Ed has said, large ones are "thought" to be able to survive on their zoollanthe alone, although I have read conflicting reports on that. As Dr. Shimek has said, these animals do not lose their gut as they grow which suggests they still need to feed. A "slurry" of squid, and/or fish would be a very inappropriate food for these animals which do require their food to be of a certain size.
DT's is an excellent food source for all filter feeders and since it is a live food, it will not foul your water.
Because a 90g is a deep tank, I would be concerned that not enough light is reaching the clam. Putting it on the rockwork is an excellent idea. And yes, they can orientate themselves quite well.
Most people will recommend MH for long term survival. I have heard of someone keeping clams for years under VHO but I have no idea if it's true or not.
Right now I have a small 2 1/2" maxima in my 20g under PC's and NO's. I already have a MH fixture but I am going to wait and change over my 90g to put the clam in. Another project!
HTH