if it is, it isn't Ca.
Senescence is hard to avoid 100%, so i'd chalk it up to that... you have plenty to finish up, so no worries.
if i did anything, i'd just feed it as normal.. check labels, if you provid plenty of Ca, you don't need to add it. give an extra fertilization instead of a water-only -- if you alternate or whatever.. just add one, don't up concentration or change mix... because, it looks fine... some fade is okay and perfectly normal.