White surface specks in sump

[acabgd]

I suppose this is some kind of precipitation but have no idea what it is. Alk is stable at 7.3, Ca elevated at 465, Mg at 1420. Np idea about pH as never measured, but the room gets windows open every day. Any ideas?
Posted a video as well as it gives a better idea.

 

[acabgd]

I see now probably posted under wrong category - still not familiar with the App, sorry about that.

 

[Dan I]

Looks like dust to me. Where is your sump located? Garage? Basement? Right under the tank?

 

[acabgd]

Right under the tank. I'm now positive it's Calcium precipitation. Measured Ca with Salifert, needed 1.04ml solution to get colour to change, meaning way over 500ppm. No idea how it got that high. Reduced dosing from 22ml/day to 15ml/day hoping it will help. Alk stable at 7.3 as it's been way back.