Very well could be. And there could also be a few gallons of old nasty water in the ladder rails. Until they are proved otherwise they are suspect.
However. Once you go milky/cloudy you have killed most/all of the algae and then it turns from a chemistry fight to a filtering it all out fight. Your FC holding overnight helps point to this. (But give the ladder and light a good once over just in case they are hurting you also).
The good news is that the FC is holding which makes this part of the battle much easier.
The bad news is this is the stage that tests even the most patient of folks while waiting for the filter to do its thing. It can drag on for a week or more, but hey !! At least you won’t be blowing through gallons of bleach
Watch the filter and clean/backwash it anytime it rises 25% in PSI. You got this. Stay the course.