I think you're describing a separate plastic strip that covers the spark plug wires along the camshaft cover (I call it a the valve cover). That plastic strip is about 4cm wide and has pegs along its length to fit into holes in the valve cover.
I imagine it's an inconsequential piece of trim that does little more than provide a cleaner look to the engine. It should be fairly simple to pry it off and see what you can do to seat it properly. No gaskets or fluids to worry about.
When the dealer replaced my plug wires, they taped some or all of the plug wires to this plastic strip at a couple of locations.
If the valve cover had a 7-8mm crack, you'd have a big oil leak.