How do I clean my iron cast grill pan without removing the seasoning?

Ok, so I grilled some chicken the other day with my iron cast grill pan and the marinade left tonnes of black residue that isn’t coming off easily with just a brush.

I don’t want to put it in the dishwasher as it’ll just rust, and won’t really clean it off any ways. And I tried soaking it the last time I used it and it did nothing (besides rust of course).

So what do I do? Do I just continue to use the pan with this residue burnt in? Is it possible to? Or do I have to brillo scrape it all off and have to reseason?

Thanks

This is basically what I have to avoid any confusion: http://common4.csnimages.com/lf/1/hash/2438/259327/1/Cast%2BIron%2B10%2522%2BSquare%2BGrill%2BPan.jpg