Overuse of Ram [CentOs 6 + Plesk 12.5]

Hi guys,
i'm italian and sorry for may bad english.
I looked for something in the forum but did not find anything.

I'm noticing that I overuse of Ram. place a screen for you to see.
can you help me?

Ps:
Server: Intel Xeon 6 core + 36gb ram + 128gb ssd
Site: There are 5 sites in the server + 3 in wordpress.