How long has it had this? If the fish has had this for a long time, it could be a neoplasm. If it showed up in mthe past few days to a week, it is probably bacterial. The trouble is that it’s location is too close to the brain. If the bacteria breach the skull, you won’t be able to save the fish.
The only at-home remedy I can suggest is to move it to a treatment tank and dose it with a broad spectrum, gram negative antibiotic such as Neoplex or Furan-2.
Jay