Long Answer: I think (could be wrong) at least some of that is virtual memory that would get reclaimed if you needed it, assuming that the resource which used it is no longer requiring it.
Observation: 1/2 GB of total memory is not a lot for a modern daw system. Your 511mb total in that screenshot seems to imply that you may not have even that much in real ram.
How much ram do you have on your system?
Short Answer: If your projects run without issues, I wouldn't worry about it.