What is the opinion of Reddit about the
Honeywell HFD360 Air Genius 6 Air Purifier with Bluetooth, Large Rooms (260 sq. ft.), Black?

This is the air purifier I use and it’s awesome with a small profile and app connectivity.
As for mold and your new mattress, when you move the old mattress out clean the area with a vinegar mix (google for tips) and consider getting some kind of bed frame to keep the mattress off the floor.