A couple times a week just slowly turkey baste some in the general area. I used to do that to my small maxima -- but I haven't done that for like oh.. a year and a half and he's been fine. Hasn't grown much... maybe 1/2" - 3/4" in 2 years? (can't remember my years anymore).
Honestly I scrape my glass daily and I've heard from several local veteran reefers they feel thats enough additional food for their clams. My derasa has grown at least an inch in the last year but he was also 5 inches to start verses 1 1/2 - 2" on my maxima. My crocea is hard to tell because his mantle covers his shell all the time
If you can culture your own Iso? (I don't know the full name of the culture -- but it's like a brown/yellow of the cultures) that's one of the best clam foods. A local guy grew his derasa from about 3 inches to around 7 inches in about 6 months with a lot of feedings of that culture (if I have it right).
I've seen the clam in another tank as he redid his... they stopped feeding it and its probably close to 2 inches larger than mine as the growth dropped off but still growing.
If you want your clam to grow faster --- feed it but I personally wouldn't spot feed it. I would just make sure some food gets into the general area via current a couple times a week. Even a small clam will get most (not all) of it's food via light source. I haven't fed my maxima in ages.. and I doubt its pushing 3" yet and it's a beautiful purple. It was also shaded by a plating coral for a long time and developed light spots on it from getting it originally under PC lighting... but that has finally all cleared up about 2 years later
Sorry if I'm rambling but Finding Nemo just came on at work so I have something to watch!