Yes, agree, if it was clear the first two days (means there wasn't too much extra dust from the substrate) and now is milky gray/white cloudy then that's a bacterial bloom. The heterotrophic bacteria are eating the excess materials used to glue the tank together (weird eh?)
You cannister filter of course is just a piece of hardware at this point and its needs its "software!" The overall filter won't be ready to handle fish until the biofilter (biological filtration is one of its 3 main functions) has been grown and tested to show that it can handle a bioload. If you are unfamiliar with that concept then you've come to the right place! Our beginners subforum handles a lot of biofilter prep each month and we have a lot of successful beginners and tanks that have "graduated" if you will.
For almost 100 years people have put fish into tanks with biofilters that are not ready yet but in the last 20 years a methodology has appeared that helps to stop this practice, keeping the fish free from permanent gill and nerve damage. If you take a look you will find lots of threads here where beginners are doing the process, called Fishless Cycling.
~~waterdrop~~