Tabs aren’t associated to any model. You’ll have to put in the model and row for the mustache.
{{#$Model.YOURMODEL.data.0.ROS_Completed__c}}sk-icon-mark-completed{{/$Model.YOURMODEL.data.0.ROS_Completed__c}}
Also, since the tab is a component that isn’t associated to a model, it won’t be dynamic to the changes happening.
You’ll have to use javascript to update these values. Here is a post about it.
https://community.skuid.com/t/associate-tabs-to-models-so-merge-fields-become-dynamic-ie-model-modelname-data-length
