The triangle icon with arrow up and down in it in notification icons as follows:
If you saw this type of triangle icon with an arrow facing up and down in it, it actually means you have turned on or enabled your phone’s “Data Saver”, which helps your phone to reduce data consumption in the background. You should not worry about this icon as it helps your phone to save data consumption safely.
Data Saver Icon May Defer As Following:
Other Android phones may display the “Data Saver” feature notification icon with 3 quarter round and plus sign inside the icon.
How to enable/disable this “Data Saver” icon?
You can turn off or turn on this feature as follows:
Settings > Connections > Data Usage > Data Saver > Toggle left to disable or right to enable it.