It’s the multicolored flower icon typically found on the home screen.  As you select photos, a blue and white check mark will appear on each thumbnail. To select all videos at once, tap Select All at the top-left corner of the screen. It’s at the bottom-right corner of the screen. A confirmation message will appear. The videos will be removed from your iCloud storage.

Summary:
Open Photos on your iPhone or iPad. Tap the Videos album. Select all of the videos you want to delete. Tap the trash icon. Tap Delete <number of> Videos.