Cloud symbol next to apps
Source:

These are offloaded apps, new for iOS 11.  A cloud next to an icon means you enabled offloaded apps and this app was deleted, but the data was saved and you can re-download the app by tapping on it.  When you tap the app, it begins to redownload to your device, as you've seen.

To change this behaviour, go to Settings -&gt; iTunes &amp; App Store and change the switch for Offload unused apps.