Rather large spots at that. If your sure it's not another animal (isopod) it was most likely Lympho. Not too sure, as you say the pics are not that clear. Lympho unto itself is not deadly though. It would have needed to be covering the mouth or fouling gill function to be a serious concern. What causes it however can be a concern.
Please post your tank parameters (actual numbers please). Any other fish in the tank and how long was the fish like this?
Cheers
Steve