Why in the prologue people are astonished by "floating rocks"? Why do they point it out?

What you do not understand is that they do not have to be close to scan it, they can try to scan it thousands of light years away if they like and have advanced scanners.

Why do you assume that it's something easily detectable from distance by scanners that aren't deliberately looking for it? It's described in-game as a navigation hazard more than once, by people who do know it's there, which is unlikely to be the case if it could be easily picked up and scanned. The "should've picked it up" argument relies on it producing effects that would be detectable, and odd enough, to be picked up by scanners looking for more general things they'd expect to run in to, that could be detected at FTL speeds. "Why didn't we see this?" is a question that should've been raised, but when you've got such a technobabble / space magic phenomena all you're going to get in reply is a technobabble / space magic response, unless a lot of thought has been put in to the Scourge.

The situation isn't ideal but since the worldbuilding stage is the easiest one to accept space magic (get your exceptions to reality out of the way early then live within their confines) I really don't see this as a massive problem.

And none of these advanced races seem to have the intelligence on placing hazard warnings for anyone approaching.
Same with our ships that came before us.
We sure have very intelligent races in the Milky Way and Andromeda.

Why in the fuck would anyone be placing warnings? WHERE would they even place them? You can't just place a "wet floor" sign in front of a gigantic anomalous mass that moves and seeks its targets. The people who did know about the scourge were already notifying each other in their own networks and working out how to navigate it, so lol wtf

that would still be a warning for the enemy. You don’t think the apozing faction would not figure these beacons would hold some meaning? Even if the beacon sends no signal to the pairing faction the other faction will still be able to scan and find them.
Your question is like ask why 2 apozing faction mark where they place land minds.

I heard them say it moves, was attracted to RemTech, but I guess I assumed it moved like an inch a day.  I saw lots of scourge-infected Remnant sites, scanning it, and it never moved.  I never saw it whip out after the Tempest, and I must have missed it in the last battle, too. 

Perhaps this could have been made more clear if it had a more swirling, active visual.

While I can concede that I may be wrong about it moving, the presentation in-game, when I played it, still made me go, "wtf?  How could they have hit THAT?" And I am sure that is not the response they wanted.

I just remember debris. And again, I never SAW it move. It didnt move at Remtech sites on the surface, in vaults, or when I was flying through it. It didnt appear to move in any of the system views. So, no, i am telling you, I played through the whole game and never got the impression that it moved so fast and so sudden it could catch a turtle, let alone a spaceship. I never felt if I got too close it would suddenly grab me before I could react. It would have only taken one sentence at the beginning for Capt Dunn to say, "Holy crap, that thing tried to grab us!" Instead the impression I got was we hit IT.

I am not denying it is supposed to move. People who read the books say they explain it there. I am just saying it seemed wrong to me, and it went on from there. (I think my next wtf was why couldnt an N7 hold his breath for 30 seconds and alternate wearing his helmet with me for four freaking minutes?)

The kett dont seem to have any problem breathing there...and by 30 seconds i meant hold his breath for 30 seconds whole i wore the helmet, the i hold my breath for 30 seconds while he wears the helmet, etc., for 4 minutes.

and again, obviously other people saw it move at the end of the game.   I didnt.  The bottom line is....it took away from the game FOR ME.  

the moment you talked about the kett not having issues.breathing there proves you were not paying attention to the plot. The kett are bioengineered to endure near any environment. Heck we even fought them in the vacuum if space in cora's loyalty mission.
The trading helmet is not going to work. They are in a vacuum. The air in there lungs are going to be suckered out.

The planet wasn't a vacuum. It had "air" it just wasn't breathable. The issue with trading the helmet was that evac was 3-4 minutes away, and you'd already breathed a large amount of technically toxic atmosphere. You were unconscious.

Well, at least the presence of a cloud cover confirms an atmosphere. 

That's fair. It's sort of the same reason I hate ME2's intro so much, so I guess mileages vary.

A big part is Joker though. Goddamn I never wanted to throttle an NPC so badly.