[USCT#1]

What are these creases all about? What causes them? Are they a good thing or what?

[slavikborisov]

Silicon glass deposits formed from slica in the molten weld pool. Cause by the added deoxidizers in the wire. If welding a critical application wire wheel them off or hit them with slag hammer. Certain mig wires cause more of those deposits than others and also deposits and also depends on how much amperage output there was.

[USCT#1]

Not the silica deposits, the creases in the throat of the weld. What causes the crease line?

[Poland308]

Just the way the weld puddle cooled. It comes from a combination of feed speed and travel speed. But mostly it only happens on short circuit.
You could try a little more of a circle motion that might help blend it in.