Is this crack on my seat stay mechanically safe?
I think the key question is: how much strength is the elliptical plate brazed over the top of the seat stay adding to the strength of the joint? I'd guess it is adding some strength, as without it there is not much material between the main seat stay tube and the brazed joint. I'd be worried about a crack forming just below the joint which could lead to the stay detaching from the main triangle

However, if one stay detaches it likely won't be a catastrophic failure, because the other stay will still be supporting the rear triangle. 

Really you need to consider the risk yourself. If you are a heavy rider who rides the bike hard there's more risk, obviously. At the very least I'd periodically inspect that area to check for any other cracks forming.