Hi and welcome to the forum
It looks like a graze/ scratch on the bristlenose's nose. Clean water and gravel cleaning should let it heal without any issues.
You can treat white spot by raising the water temperature to 30C and keeping it there for 2 weeks, or at least 1 week after all the white dots have gone. It's safer than using chemicals and if you have discus, the temperature is probably pretty close to 30C anyway.
Don't use heat and chemicals at the same time.
Before you raise the temperature, do a huge water change and gravel clean to remove as many parasites form the tank as you can. Make sure the new water is free of chlorine/ chloramine before it's added to the tank.
Clean the filter too.
Increase aeration to maximise the oxygen level in the water.
Insulate the back and sides of the tank with 1-2 inch thick polystyrene foam.
Have a coverglass on the tank if you don't already have one.