I tried to separate soldering on a Raspberry Pi but exposed some copper. The Raspberry Pi boots just fine. Is this safe?
That copper is most likely the ground fill unless there was a trace there before. If there was a trace there as long as you did not damage the trace you should be fine (if it doesn't touch anything that wasn't already connected to that trace). If its just ground (what it looks like) you should be fine as long as it doesn't touch anything that's not ground.
If it isn't touching anything you should be fine. If you're still worried about it just cover it with some glue to make extra sure it's insulated. Its right next to exposed pads anyway though so thats probably not necessary (anything that would touch that part would touch the adjacent pins as well).
Just make sure if you melt the solder on those pins again that it doesn't jump and connect itself to the exposed copper (as it looks like the exposed bit is pretty close to two of the pins). Also double check it with a multimeter to make sure you haven't already done that... just check to make sure the exposed part is ground and unless either of those pins are ground you should not be reading continuity between either of the pins and the exposed pad. If its not accidentally connected now and you leave it alone it should be fine.
You can tell from this picture that the exposed copper is ground:
So all you need to do is test with a multimeter to make sure neither the 4th pin from the bottom (of this picture) or the 5th pin from the bottom are connected to that exposed spot. If they are not then you are all set (put some glue on it for ease of mind if you want). If they are connected then it is most likely a solder bridge; use a desoldering wick to disconnect the pin from the exposed copper. From this picture though (although it is blurry) I do not think it is bridged.