You can destroy a face-down monster with “Newdoria”.
Netrep Rulings:
The owner of “Newdoria” decides which monster will be destroyed by “Newdoria’s” effect.
“Newdoria’s” effect is mandatory.
GoatRulings.com Rulings:
Q: If “Newdoria” and “Pyramid Turtle” crash or destroy each other in battle can “Newdoria” destroy the monster summoned off of “Pyramid Turtle”?
A: No. “Newdoria” is a mandatory effect and “Pyramid Turtle” is an optional effect so “Newdoria” will be chain link 1 regardless of who turn player is. Newdoria must target at activation. So the target to be destroyed by “Newdoria” will need to be chosen before the effect of pyramid turtle activates.