Kodak's book "Conservation of Photographs" gives a procedure for removing dichroic silver stain: a ball of absorbent cotton is dipped into a solution of ammonium hypo reducer and applied (rubbed?) in a cicular motion. This is shown in a photograph, Fig. 93. The ammonium hypo reducer is prepared by adding 15. grams of citric acid per liter to Kodak Rapid Fixer with hardner diluted to 1:3. It would be very wise to try this on an unimportant negative.

As to the likely cause, Table VIII-1 lists various stains, including "appearance of a metallic sheen of various colors. Usually bluish to brownish and often called a tarnish". The explanation is "silver stain resulting from oxidation of the silver in a non-image area" and the cause "incomplete removal of silver complexes". This doesn't quite fit your description since you see the sheen in the highlights. The appearence of dichroic silver is described as "a non-image stain yellow by transmitted light and grayish by reflected light".

After cleaning, refixing and rewashing sounds like a good idea.

You might want to buy this book or find it in a library.