That appears to be calcium deficiency, definitely.. Keep in mind that excess salts from earlier feedings are known to lock out calcium. I happened upon this issue several times around the same time of flower.. It would often be from lingering salts from the Nitrogen in the earlier veg nutes not being flushed completely from the root zone, or just excess feeding at the end of veg/beginning of flower.
I'm not familiar with that cal-mag supplement but it seems you could bump it up easily for the next feeds.
Personally, I would also go ahead and leech/flush the soil immediately in case there is lingering salts from veg. I would first check your run off EC/PPM real quick to tell if there is excess built up of salts, than you can make your decision
