Can a recent aquascape cause these problems?

I have a 20L lps tank that for the last 5 months has not necessarily thriving yet it didnt have any visual problems except a few species of lps that did slightly recede. I recently had to change alot of the aquascape to retrieve a sleeper goby and peppermint shrimp out of my DT. In the process i had to move a couple of the rocks that were on the sandbed, causing some milky white substance in the water column near the base of rock, some chalky dirt looking detritus fell out of certain rocks aswell

Could this be the reason for my ph levels and alk levels to drop in my tank? Im trying to figure why my calcium and magnesium stay stable but my alk drops sometimes all the way to a 6. I keep using buffer (that says also raises alk) and ph will still not go past 8. Most of the time it reads 7.8. I have cyano starting to form on sand/overflow box. The last day or two small bubbles have been forming on live rock and a rust colored slime on the sandbed. Im taking precautions incase it is dinos, siphoning what i can see and i turned lights off yesterday, left the 6500 t8 on my chaeto in sump. What would be your best approach at stabilizing ph/alk and what should be done in terms of water changes and should i buffer my top off water from now on?

Salt mix is aquavitro salinity by seachem

Any help you can give is highly appreciated