When you start the Volumio web GUI, there’s some flickering going on with raw variable names in placeholders briefly being shown in the UI.
I’ve noticed that AngularJS is the framework used to implement the Volumio GUI. Flickering is common with Angular. The solution is to use the attribute ng-cloak; this can be done on the element.
And by the way, in the future you may want to migrate to Angular2 which is much, much faster than AngularJS.