These plastic tanks have a habit of leaking around the breather seals and the pump gasket. They were subject to a safety recall, but it may not have been done (mine hadn't)
Repair involves dropping the tank and skid, replacing the seals and the tank straps that you will have snapped the studs off on removal, and refitting.
Or, take it to a main dealer and get them to check via the chassis number if it was subject to a recall, then let them get the grazed knuckles/ crap in the eyes/ raging arse with it. For free
The skid is thick steel, not the usual tin so I would imagine it's just surface rust anyway.