I have had this happen on multiple occasions. It only happens when I use the Carbon A from SR and I cannot figure out the reason as to why. Kind of looks like ash sometime but almost identical the pictures you posted above. We filter through a .45 micron glass paper and that does the trick. I thought it was silica as well, but when I removed that step we still had the particulate present.
Ours also dissolves in water as well. Quite the problem and unfortunately I do not have any answer, but you are not alone.