You have some great points but it's my belief that if a clam's mantle grows as the clam does it also increases it's surface area. In doing so it allows more Zoox to inhabit it's inter mantle thus causing more and more photosynthesis thus allowing the Zoox to release more and more by product into the clam as the clam grows.
People have always jumped to the conclusion that small Maxima clams need to be fed or they'll die. Over the years I've came to believe that this is not the case. I've seen tons of instances where someone says "why is my clam constantly moving from it's spot", and then you'll usually see someone reply "are you feeding it?". I firmly believe if the thought that a 2" or less Maxima needs feeding then why are all the millions of Maxima clams in the oceans around the world still alive? I mean, our tanks are complete septic tanks compared to the water conditions found in the wild reefs.
It's my belief that small Maxima clams don't usually die from being underfed, rather than they are extremely easy to stress out causing death by the aquarium keeper fussing with them too much. I've even done some tests on my own with small Maxima clams, now are they scientific? Not by any means but I do know what I have found and what works for me.
You may or may not have seen this photo kicking around the net, it's pretty popular and I got tons of comments on it when I originally posted it. The Montipora Cap in the picture was bowl shaped, and all the clams inside of it were 2" or less. All three were quite mobile and I couldn't ever get them to stay put in the places that I chose to keep them. After messing with them for about a week I decided to place them in the Cap and see what they did, since it was bowl shaped they couldn't jump out and within a few days they actually attached to it. If memory serves me correctly over a period of about a year they grew to about 3" without any supplemental feedings. I'm sad to say that I did loose the clams though, I made the mistake of adding three clams that had a virus.... Bad mistake, it wiped out all of my clams within a week or so...
So, do I currently feed my clams? No, nor will I.
Do I think that rjcranor's clam is doing what it's doing because it needs to be fed? No, I believe that maybe something has changed in the tanks environment causing the clam to no longer like it's current placement on the rock. I haven't a clue what the change could possibly be, it's not my tank so I don't have the ability to watch it day to day. My first guess is the algae is in some way causing the clam to want to move, weather it be just that it's irritating it or the possibility of a fish or invert picking the algae and irritating the clam. I would like to see a clear top down pic of the clams mantle to see if I can see any bleaching. If there is indeed bleaching then I would tend to think ether it's getting too much light or not enough. From the pic's above the mantle is not apparently reaching for light so that leads me to think it's getting enough light.