The white pimple stuff is basically a cyst pushing out of the skin, which starts they HITH. Also another symptom of HITH is the line that you see that travels from the top of the gill and straight back towards the tale. Your picture shows this. Basically it’s the nerves pulling apart.
To treat this you will want to QT the fish and use a drug like API General Cure (APIGC). You will need to treat him with APIGC at least 2-3 times.
The way HITH starts is due to poor water quality and/or poor diet, like bad food.