Yeah those types of clips can be a pain, especially when they corrode together. You can try spraying some electrical contact cleaner up there, let it sit for a few minutes and then wiggle it like Andrew mentioned. It also usually helps if you push it "on" further, which breaks any of the bonds, and then try pulling it off.

I havent gone this route yet but I believe Adam finds it easier to access the starter through the wheel well. You'll just need a $5 box of plastic rivets and the install tool to get the liner back in.