Yesterday, Twitch updated their chat on mobile devices for users to use mentions. This is great addition to Twitch Mobile for the on the go users who aren’t by their computer all the time. Make sure to update your app in the iOS or Android store to use the new features.
Starting today, you’ll be able to use Chat Mentions on both iOS and Android. Just like web, when you mention someone in chat, that name will be bolded in the mentioned user’s selected chat color.
To mention someone, type @ or tap on a username within the chat window. From there, you can choose to mention (happy face) or ignore a user (sad face). If you’re the moderator of a channel, you’ll also see options to time-out or ban someone. Note: You can still moderate using the viewer list.
Via Twitch Blog