cub cadet trans axle leak

I have a 2015 lt42 cub cadet and noticed a small oil leak and traced it to what looks like a relief valve on the left rear side of the top of the trans-axle
I touched the black cap and it fell of showing a small circular rubber part which i am not sure what to do next with. Is the cap a one piece and now broken or is it supposed to just sit on the top? does this all come off somehow so i can replace the relief valve?

That is the breather do not touch it it is there to vent the transmission.
A little oil coming out is no problem, just shows you have been driving at steep angles or working it very hard in very hot weather.
Clean the oil off so it does not become a dust cover which will affect the cooling of the tranny.
Check the fan is connected to the input pulley come times they break free and the box overheats.