Every single video composition I create has, by default, a #37b0c9 background. It's a color I use frequently so I'm worried it may have been a setting I entered somewhere by accident, though I've scrubbed through all my settings, to no avail.
This also carries through to rendering. If I'm using a pre-comp within another larger comp, and the precomp has any 'empty space' in the beginning or end, it fills the entire screen with #37b0c9, even if there is a background or video playing under or over it.
Press Ctrl/Cmnd + K and show us the composition settings.
If that does not work then mash down all of the keys on the left side of the keyboard (Shift + Alt/Option + Cmnd/Ctrl) when you open AE and have AE delete your original preferences (or find the folder in the Preferences panel and delete or rename the old preferences file and restart)
Thank you so much! This was the issue.