It can mean this a lot of the time, but not always. It can also grow with a substrate-source of nutrients. If sand or rock has enough detritus on it or has absorbed enough phosphate, it will grow cyanobacteria regardless of concentrations of nutrients in the water column.
Been there man...and dear god was it a nightmare when I was new to this. 50% water changes etc...
Ill spare you the dramatic story of all the crap I went through trying to stop it. Lets hope he doesnt have that problem.
That means sand change time and phosban needed bad.