What's wrong with this white cloud minnow?

Can anyone tell me what is wrong with this fish? It has suddenly developed a purple spot on its side. I've caught it sitting on the bottom a few times and it looks a bit pale. Most of the time it's pretty lively ... especially at feeding time. I've had it for about 2 years.

It's in a 34 litre tank with 3 platy and 3 other WCM, so plenty of room. I've not added any new fish to the tank recently, and the rest of the fish are perfectly fine.

I'd be grateful if anyone has seen this before and can tell me what it is.

Thank you.

Sorry but can we have a better picture?

It could be a bruise or the start of a bacterial infection.

Wipe the inside of the glass down with a clean fish sponge.

Clean the filter if it hasn't been done in the last 2 weeks.

Do a 75% water change and gravel clean the substrate every day for a week.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

Add 1-2 heaped tablespoons of rock salt, sea salt or swimming pool salt for every 20 litres of tank water. Keep the salt in there for 2 weeks.
When you do the daily water changes, add salt to the new water before adding it to the tank so the salinity (salt level) in the tank remains stable.

If it gets worse, post more pics.