Whatever it is the frame is steel and won't really fail dramatically anyway. I wouldn't stop riding it but do as you want.
Looks like it too is just welded on top, just some paint is covering the seam so it hasn't rusted yet.
E: so in other words. RIDE ON BROTHER!
Looks more like a spot they didn't weld at all. Probably fine but check with brother first.
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Whatever it is the frame is steel and won't really fail dramatically anyway. I wouldn't stop riding it but do as you want.