An update after one month, hope this helps others.
A strange thing I noticed a couple of weeks before the first bubbles apperared was the skimmer overfoaming and overflowing for some hours after a water change.
This continued for other 4-5 water changes, until I checked the freshly prepared salt water and I found Mag to be out of scale!!
In the meanwhile, Mag in DT was 1,800.
I then mixed many many times the powder in the bucket, made again the salt water and the Mag was 1500 as it shoud be with that salt.
So my clue is I had a "layer of Mag" in the salt bucket that caused a spike in Mag over a few water changes, and for whatever reason only the green stylophora had been stressed.
The lesson learned is to mix many times the whole bucket powder content prior to using it.