If ya can't find the source, then it will be reoccurring. It could be run off or rain or many other variables. Might be a result of the algae dieing and decomposing after using the Algaex product. After the very big water change, the massive amount of chlorinated water might have killed quite a bit bio-film creating the residue; dechlorinators work good, but they are not instant. It is better to add slower, smaller water changes over a period of time rather than one big water change.
To resolve the murkiness, you can try a pond safe flocculant or polymer product if looking for a liquid product, but I have never used any of them.
Also, can get some quit batting in a container and allow the water to get filtered by it for a few days.
There are also some natural polymer products out there. One particular produce is
Koi Clay Clacium Bentonite. Here are a couple threads talking about it: 1)
pond calcium montmorilloniite clay; 2)
ultimate koi clay. I think this works due to a particular polymer released as the clay product dissolves and absorbs into the water.