I'd have to see it closer, but it looks like the braze didn't completely fill the steerer/crown joint. Water will or did wick into the gao and start a corrosion process. It's not a good scenario, but the fork may still be sound if the bulk of the joint is OK.
Not seeing it, closely I can't give you a solid opinion, but based on the photo, I'd take steps to stop the rust process, and seal/fill the gaps and keep riding. But as a backup safety measure I'd keep a stainless 6mm bolt in the brake hole full time. That will keep the crown and steerer together if the braze fails.
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