My goldfish has a white/purple bubble on its forehead what is it?

Mallory try and give us as much information as asked for in the box above that way we can do our best to help you

Ill try
I dont know the test reasults
5 gallons i dont know how long
no filter because its one goldfish
I change the water and clean the tank once every month
1 fish size 2 inches
none
no
no
green fish food pellets and dried shrimp sometimes
no except the bubble on its forehead
no

Given your type of Goldfish you would need a 10 gallon tank.Goldfish are also very messy so a filter would be a good idea.You need to do regular water changes of a minimum of once a week.If you take your water to a LFS they will test it for you and write down the results.Does the bubble look like an open wound.If its an Oranda it could be wen growth.

No it looks like a little pimple on his forehead and its white/purple.

Hi Mallory,5 gallons is really small for a 2 inch Goldfish.As Blue mentioned,Goldfish should have at least 10 gals. per fish,along with a very good filtration system as they really are messy.
With it only being a 5 gal. with no filter,you should be doing water changes a couple times a week-not once a month.His water is very toxic at this point I'm sure.
For now,I would do a 25-50% water change,and then invest in at least a test kit so that you can keep an eye on his params.If you can't do that right now,then as Blue said,your LFS will test it for you.
How long have you had this goldfish?
Are you able to post a pic of this bump?