Removal of house numbers on stucco!
Probably construction adhesive. The main problem is that this doesn't come off cleanly. On stucco, this is likely to turn into gouging out the letters, glue, and the stucco underneath, then repairing the stucco. Figure out if this is acceptable now.

You can use a hairdryer to heat up the area to soften the glue. There might be screws underneath, used for creating a good surface to adhere to.