Can you post a couple of pictures of the entire fish?
If looks like excess mucous, which is normally caused by poor water quality or protozoan infections.
What is the ammonia, nitrite and nitrate levels in the tank?
What is the pH of the water?
How long has the tank been set up for?
How often do you do water changes and how much do you change?
Do you dechlorinate the new water before adding it to the tank?
What sort of filter is on the tank?
How often and how do you clean the filter?
What other fish are in the tank?
Have you added anything new to the tank in the last couple of weeks?
--------------------------
Male goldfish can get white spots on their gill covers and the edge of their pectoral (side) fins when in breeding condition. Female goldfish do not get any spots, but they get fatter from the eggs developing in them.
--------------------------
Test the tank water for ammonia, nitrite, nitrate and pH and post the results (in numbers) here.
Wipe the inside of the glass down with a clean fish sponge.
Do a 75% water change and gravel clean the substrate every day for the next 2 weeks or until the problem is identified or resolved. Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, do not clean the filter if it is less than 6 weeks old because you can wash out the filter bacteria and cause the tank to cycle again.
--------------------------
The following link has information about what to do if you fish get sick. it's long and boring but worth knowing. I recommend printing it out and reading it in bed to help fall asleep.
https://www.fishforums.net/threads/what-to-do-if-your-fish-gets-sick.450268/
