no that is not ich , or "white spot " . my sd's get that sometimes if i slack on some water changes , bet then it'll get better when i ramp up the maintenance .
It looks like it could be a type of fungus. Some of my silver dollars have gotten it. By the time I managed to curing it their whole tail was gone. It did grow back though. Up your water changes and keep an eye on them. If that doesn't do it you may have to medicate. If you have any scaleless fish in your tank you may want to put the Silver Dollars into a different tank if you end up having to medicate.