Im assuming your new filter is replacing an older filter, and prior to this it was an established, fully cycled aquarium? If so, when replacing a filter you should be moving your established filter media from your old cycled filter to your new filter.
Its a bacterial bloom. By replacing the filter you have lost denitrifying bacteria which is responsible for your cycle, and a different type of bacteria is proliferating. Rather than consuming ammonia and turning it into nitrate, this different bacteria consumes ammonia resulting in growth until you can see the bacteria in the water. As your cycle re-establishes the ammonia this bacteria is feeding off will get cut off and the bacterial bloom will end.
Do you know your water parameters? From your other posts you have done a few water changes recently, so knowing what they were before these changes would be more useful.
Water changes will remove the nutrients available to the bacterial bloom, but until your cycle re-establishes and denitrifying bacteria outcompetes your bacterial bloom its just a matter of waiting it out.
Do you know how to cycle a tank?
