A problem with brachycephalic dog breeds, especially pugs, is that they can't display normal calming signals. This is from lack of learning them in the first place and physical limitations. Your GSD may not know that your pug is trying to say "enough." The good thing is that there is no blood, so they aren't serious... yet.
Because this is happening so frequently, I would take a direct approach. I would put a dominant dog collar on the GSD and have it wear a drag line in the house. When the GSD goes after the pug, calmly say "no" and lift her off the floor a couple inches with the drag line until she settles and then put her right back down. It's really important that you stay calm.
This type of correction will not amp the dog up further and takes the power away from the dog immediately.
Because the dogs are not aggressive all the time, setting up a DS/CC training scenario is going to be very difficult.
If the dogs have solid OB, as soon as you see one of them becoming aggressive, the moment the thought enters their brain, place them in a down or recall one of them. You have to really be on your toes to read the dogs in time.
When a dog saves the life of a man, it becomes clear that partnership knows no bounds.
Fama - T.E.D.D. OEF XI-XII (GSD)(RET)
Marshall - T.E.D.D. OEF XII-XIII (Lab)(SF EDD)
Lucian - Med Alert / Mobility SD (Cane Corso)
Pud - the old man (Pit x Lab)