All the white areas of my display are a rapidly flashing pink grid! What did I knock loose?

I have a mid-2010 Macbook Pro 15-inch. I installed a Seagate 1TB SSHD into the optical drive slot a few months ago. It stopped working so I removed for return. I closed the thing up, turned it on, and suddenly (almost) all white spaces on my screen are now pink! A flashing pink grid, to be precise. For example, white icons on my desktop have a few flashing pink artifacts, and most of the white areas on pages in my browser (Chrome) are almost entirely pink.

I've opened the laptop many times before to install ram, HDDs, etc and I've never had any issue. FWIW, a few months ago the logic board was replaced by Apple due to the infamous graphics switching error.

Has anyone seen anything like this before? Could I have knocked something loose? Touched the wrong thing? Shocked the logic board?

Any help is greatly appreciated.