I notice this in my fish and i dont have idea what is it first is like a little bubble and burst like two weeks ago. Is doing really good and everything but i dont know is this can be bad.
That looks like an injury. It doesn't look very red. If it were any other fish that eats well in quarantine, I'd suggest removal and treatment with antibiotics because I worry about infection.
However, it's tough to feed mandarins in quarantine and in this case I'd leave it be. Keep water quality high and parameters strong.
Looks like a cyst has popped. When it did it "potentially" released thousand of parasitic spores into your tank. Not much you can do other than to boost your fish's immune systems. Pristine water quality, high quality foods, consider adding Beta Glucan to your foods. It boosts immune systems of fish incredibly well. Info below on BG. And as @4FordFamily posted watch the spot for redness, or other color changes or necrosis at the site.
Wouldn’t it be great if there were one thing that we could give any sick fish that could help it? Aquarists dream of a “magic bullet” that could be used as a treatment for a plethora of problems.