It's called a biological bloom and it's bacteria consuming nutrients in the water until they grow in such numbers that they become visible.
Bacterial blooms are common in newly established aquariums, and they tend to go away on their own as your cycle establishes and the nitrogen cycle consumes these nutrients.
Is it just the fish you inherited and you got new equipment? Or did the aquarium, filters etc all come with the fish?
How often are you doing a water change and how much do you normally change?
There are 3 issues that could be the cause. And it might be a little of all 3.
- You arent cycled. If it's a new aquarium then it needs to be cycled. If it's an established aquarium then if it was moved it may have disrupted your cycle and it needs to re-establish.
- By "change the filter" i presume you mean change a cartridge? The microbes responsible for your cycle live inside your filtration, and every time you change anything you throw those microbes away, which again disrupts your cycle. Don't throw away cartridges unless they are literally falling apart. Just rinse them on dechlorinated water and put them back. When you absolutely have to change a cartridge, don't change them all at once. Change one, 2 weeks to a month later change another. That way you arent throwing away all those microbes and your cycle doesn't need to catch up so much.
- Your aquarium is too small for that size fish. You really need something at least twice as big. There simply isnt enough water volume to sufficiently dilute the waste your fish produces. You haven't mentioned what filters you have, but they not be enough to filter out all the waste such a large, messy fish.
Do you have a water test kit? Can you test your water parameters?
I'm conscious you might have no understanding of what the nitrogen cycle is, or what cycling a tank means. So please let me know if I have used terms you don't understand and I can explain in more detail. But its going to be a little science lesson.
It looks like a goldfish. How big is it?