Brown liquid mixed in with the antifreeze?

I have a 1991 Mercedes Benz 190e and I was checking my antifreeze tank one day and saw that it had turned brown or had brown stuff in it. I had it checked at a mechanic and he said it was my radiator that was bad so he fixed it. A week later I saw that the brown stuff was not gone so I took it right back to the mechanic and he had a liquid called blue devil which is a head gasket sealant cause he said that it might had been my head gasket moved when my car over heated a few weeks ago. But just yesterday I looked into the antifreeze tank again and saw that it had a little bit of that brown stuff in it still. The mechanic told me that it was nothing. But i'm concerned can you please tell me what this means. Because I checked my motor oil and it's clean no water or nothing in it.