That is a multi-port valve, sometimes called the main valve. Black body, white cover, black handle.
I can't be sure what I am seeing in the pictures, where is water leaking out from? Is it leaking from that round opening on the side? That appears to be the waste port, the post water come out of when you are backwashing the filter. Leaks from there are almost always caused by a bad spider gasket. You can fix that yourself if you are at all handy.