It's easy enough to verify where the SQL is running by checking the AX 2012 Server Configuration in Administrative Tools. Most likely it is running somewhere else and the SQL Server process showing in the screenshot is not used. It could be a SQL Server Express instance, perhaps used by a backup solution.
I also see from your screenshot that you also have both SSRS and a large IIS process running on this same server. (Enterprise Portal, perhaps?)
You also have Management Reporter running.
I have not found any reliable way to free up the memory taken by the AOS process, so a service restart will be needed recommended if you really need to release that memory.
Is the current memory consumption causing your system to slow down? Try to recycle the IIS worker pool, that may free up about a gig, at least temporarily. Same with SSRS, try to restart that service, if you can. But again, that is only likely to be a short-term solution.
As Martin alludes to in his answer, running all-in-one is not a recommended configuration for a production environment, so this certainly looks like a development box where you should be able to stop and restart services pretty much any time you want. If it is, you can run on a single server, but you'll need lots of memory. Heaps... Lots and lots... Otherwise it will just be too painful.
As you are worried about restarting, I'm going to assume for a moment that this is a production environment. In that case, please split your services out to different servers. SQL Server and SSRS can run on the same server if you are concerned about SQL licensing costs, just make sure there is enough memory in that box to serve both. SQL Server is designed to take as much memory as it can get, so if you are going to co-host it with anything else, set the maximum memory limit through SSMS to make sure that the operating system and any other services (such as SSRS) will have enough memory to run. The AOS should not be on the same server, and MR2012 really should have its own server as well.
Enterprise Portal can be a real resource hog, so move that to a separate one as well.
The official recommendation to split out the different services to separate servers is not just because Microsoft wants license revenue, there are real performance reasons for it. Just make sure the servers have good, fast network connectivity between them. As fast as you can go, and with low latency.
