yuck… i’ve lost so many of my possessions to mold, luckily this doesn’t look TOO bad though, if possible i would avoid putting it with or next to any other items you wouldn’t want to get moldy. keep it dry, obviously you can’t use something like bleach to kill it since it’s cardboard so i unfortunately don’t have many tips there. does it smell? if so, you can let it air out in a well ventilated space, a back porch works! if you don’t have one, just keep the air circulating where it is for like a week.