Yes that is the Hydraulic tank and breather,those parts are relatively inexpensive,dont have the part # for them but is just a plastic breather cap and the dipstick is plastic also that attaches to the bottom of the breather,this tank being empty is a wonder you have any hydraulic functions at all.
At the bottom under the truck should be a drain plug(s) for both these tanks so you can drain them. The Hydraulic tank you can fill and shine a light into the tank and level should be about 3/4 of the way full,leave some room and dont fill it to the top.