Can someone tell me what this is. BTW this is not my engine bay, oh and forget teh bits highlighted, already on ht eimage.

The bit im on about is the unit just to the bottom right of the expansion tank (looks like its touching it in the pic). Its a green coloured top. When you look at it closely the cap has an arrow and it looks like it has 2 positions. What is it and what position should it be in. Mine looks like its leaking as there is oil on the top of it. Is it the power steering resevoir or something?
Cheers,