The only difference between the two photos is the angle. The first photo was taken from directly above the clam, and the second photo was taken through the aquarium glass. What causes such a drastic change in the pattern and color?
The one word answer is "iridophores," but there's a lot more to it than that.
In an article for TFH magazine, Professor James Fatheree M. Sc. wrote, " the perceived color/pattern of many tridacnids’ mantle can change drastically depending on the viewing and/or lighting angle, particularly in the case of Tridacna crocea and T. maxima. In other words, if you change the angle that you view a clam from, or change the angle of the light source, or both, you will often see different colors being reflected off the mantle. Sometimes this isn’t very pronounced or doesn’t occur at all, but in some cases a specimen can look completely different when viewed from the front instead of from above.
Some specimens can also have a brilliant reflective sheen when looked at from certain angles, as well. This isn’t caused by the pigments, but is instead due to the reflective iridophores in the mantle."