From what I can tell grindrguy makes mods for grindr using xposed framework, things like how to get free paid memberships for the app etc. It's quite possible that there have been mods that have unknowingly made configuration changes on your device, sent messages on your behalf, deleted messages amongst other things. It seems Grindr would be the perfect gateway for this kind of infiltration to happen as many users will be less likely to ask for help on their devices when the initial vector of attack is Grindr.