I Hit My Knee In A Metallic Stair And I

I hit my knee in a metallic stair and I thought that nothing will happen as I was wearing a pant above the injury and thought no infection will occur.But after about 4 days I saw a greenish layer in the wound and it is a little deep.it is not liquid but is in solid state.please tell me what to do.Does it mean that I am seriously in a bad state or should I not worry about it.