As its softies that are suffering you can forget Ca, probably nitrates too.
Your Top off with Kalk / Limewater is most likely the problem, you’ll be causing KH swings that will upset your corals, just stop and use RO/DI water. Your KH is more than high enough anyway, so allow it to drift down to 8 and keep it stable at that level.
Other issues could be lighting or flow, and longer term high phosphates, but for softies its not going to have a sudden impact like a KH swing.
Low nitrates isn’t going to be your issue with a green algae growth – I doubt it would in any circumstances, but if you’ve got green algae you’ve got nitrates even if the test says zero.