plugs / epoxy rust?

[SeeFu]

Does anyone know what this is that's formed between my frag plug and epoxy? It's the color of rust.

I was relocating this frag since its been slowly dying and i wanted to remove the old epoxy from the plug and this is what was found sandwiched between the two.

 

[vpierce3]

I have no idea. But here’s a bump.

 

[SpyC]

Had the same issue with plugs and brs glue. No clue what it is but no issues with my tank so far.