Yes — corn is safe and universally loved by parrots, though it's starchy enough to count as a treat-vegetable.
What it means for African greys
There's nothing specific to a african grey that changes the verdict on corn — it's treated the same as for parrots generally. Greys can take a larger portion than small parrots.
The full picture on corn
Fresh corn is non-toxic, easy to digest, and almost no parrot refuses it — which is exactly why it needs rationing. It's high in starch and modest in everything else, so it earns its place as motivation and enrichment rather than a diet staple.
Fresh on the cob (cut into wheels) or frozen-thawed kernels.
Plain only — no butter or salt, so no shared movie corn.
A few times a week, not daily; it's the dessert of vegetables.