I see nothing suspicious here. Only 7.1% of memory usage.
For example, on my test Plesk Onyx server I see:
[root@ppu17-0 ~]# free -m
total used free shared buff/cache available
Mem: 3072 424 189 135 2458 2518
Swap: 512 101 410
How it is affect of your Plesk and websites operation? Why do you think that it is a problem? Do you have any alerts/notifications from Plesk Health Monitor?