Summarize:

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.
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.