Looks like cal mag yes, try to get some CALMAG or epson salt mix well and adjust pH accordingly. Shake well before each use. For CALMAG Add 1 ml per liter (5ml per gallon)of water, mix well and adjust pH to 6.2-7.0 before applying.
Regarding underwatering, maybe, i mean you are giving less than a liter in 3 days, i normally dont put specific dates, i give them when they need, so when i see the soil getting dry, and i give them enough, until the water start to drain