Urgent Fish dying

This morning one clownfish had died.
It had this lesion on its side.
The people at the shop it was from said it was just a cyst but it doesn't really look like that.It was a red lesion with yellow looking area on top.
Does anyone please know what it was that caused this or if other clownfish is at risk?
Thanks

 

The redness is indicative of a bacterial infection, and the lesion itself suggests that it may be a Vibrio infection.

I suspect that your clownfish may have been an isolated case, but keep an eye on the other fish. Vibrio is pretty much everywhere, but stress or open wounds can cause it to flare up.

Furan 2 would be a good med.