Yes, I'd say that that's fowl pox. Your other birds might not have it yet because the disease has not spread yet, or they have not been bitten by an infected mosquito/flying insect.
There is no treatment for Fowl Pox. It will run its course in three weeks or so. However, treating your bird's area for flying insects will help prevent further spread of the disease.
Antibiotics will help prevent secondary bacterial infection due to stress. Terramycin or other types of oxytetracycline are good antibiotics. Giving electrolytes and probiotics will also help with stress (just don't give probiotics when using antibiotics).
To make the birds more comfortable, you can try to remove the scabs. Put some petroleum jelly on the scabs, which will soften them. This should aid you in removing them.
A vaccine for Fowl Pox is available. If the disease becomes more prevalent (due to an increase in mosquitoes, etc), you could consider vaccinating in the future.
Hope I've helped!
