Well, last winter I was having almost zero heat, so I flushed out the heater core and used CLR ( In Canada it's some Calcium, Lime, Rust and crud remover with many uses )to remove the rust and crud in there, many people do it here and was recommended by many and it got a good flush out at the time, it could be possible there was still some left in the HC and it's now in the system cleaning as it goes !
I am the 3rd owner now for 3 years, it did it in the beginning, but not nearly as much.
I flushed out the rad, not a full system, in May and added new coolant. I read on this forum that the red coolant wasn't used on 4rnr's until a few years after '97 and that the green stuff was safe to use.
The PO never said it had any stop leak added, it's only really got like in the photo since the flushing I did this year. So if you guys are saying it looks like some kinda stop leak then it's very possible it is from the CLR I used !