Welcome. It sounds like a bacterial bloom which normally clears up in a couple of days.
At the moment, you are probably following the advice of the chappy in the fish shop, having set up your tank and filter, and run it for a few days, and then it'll be fine. It won't.
Your filter needs to build up 2 colonies of bacteria, to process the ammonia produced by your fish, via nitrite, into nitrate. There is only one way that those bacterial colonies will grow, and that is to feed them ammonia. No amount of running dechlorinated tap water through the filter will grow bacteria.
There are 2 ways of providing that ammonia. The first is the way that most fish shops recommend, and that is to buy some fish. In this way, the shops get some more of your money, which is why they like it. Fish produce ammonia as part of the respiration, but it poisons them. The bacteria colonies don't grow instantaneously, so whilst the colonies are growing, the fish poison themselves, so it's up to you to stop that happening, but doing large daily water changes. And frankly, that's a bit of a chore. And a lot of people don't like doing their chores, so they can be a bit lax, and sadly, this sometimes mean that their fish die. So they go and buy some more. If they are particularly lax, they can lose multiple batches of fish.
I mentioned a second way, and that is to obtain some bottled household ammonia. There is some available on Amazon at the moment for £3.99, with free delivery. I mention this as it has become very difficult to obtain ammonia in shops, as it can be used by terrorists for bombs. Apparently, terrorists don't know how to use the internet. But I digress.
By adding bottled ammonia, there are no fish to be poisoned, you don't have to do daily water changes, and when it's completed correctly, you can go buy a full stock of fish, you don't need to add the fish bit by bit. The down side is that it takes a while for your nice new fish tank to actually have any fish in it, which can be a bit embarrassing when the mother-in-law comes to call, but actually it's a helluva lot easier, for both you and the fish. Which is why pretty much every experienced fishkeeper on the forum will recommend this method.
I hope I haven't blown you away with too much info. Any questions, please feel free to post a query in our "New To The Hobby" section.