In a comment on the earlier article Cooked Chicken "Goop", Alice wrote:

I marinaded some chicken breasts the other day in a yogurt marinade (that I've made before with no ill results). My ingredients were all fresh (mostly yogurt, shallots, garlic, that kind of thing) and it looked whitish when I was done, as one would expect. I took the chicken out of the marinade and baked it, although obviously some marinade was left on the chicken.

But when the chicken was finished, it looked like it was covered in moss. The bits of marinade that were left turned sort of blue/green.

Do you think this was a result of this chicken goop reacting to the marinade? Or did my chicken somehow take a very wrong turn?

(I admit I scraped off the mossy parts and ate some. It tasted fine and I didn't feel poisoned, although I haven't been brave enough to touch the leftovers.)

There are a couple of possibilities for what happened.  Without actually seeing the final dish, it is hard to say for sure.  The "mossy" appearance is hard to understand, but as for the color, garlic is said to turn blue-green under the right circumstances.

Some sources suggest that sulfur in the garlic may react with trace amounts of copper found in the other ingredients, or in the utensils used, to form copper sulphate which has a turquoise blue color.  Only a very small amount of copper is needed to cause this to happen.  Generally, these sources agree that the amount of copper involved is so slight that it will not cause harm, although I wouldn't recommend taking the chance.  See  foodsafetysite.com or What's Cooking America for more information.

Another possibility is that the food was covered with aluminum foil which came into contact with the acidic ingredients.

When most metals come into contact with an acid, they will dissolve.  Depending on the metal, the strength of the acid and other factors, only a small amount of metal may dissolve or the entire piece may be eaten away.  Yogurt contains lactic acid which is what makes it sour.  Recipes using yogurt as a marinade frequently also include lemon juice, too, which is another acidic ingredient.

If the cause was aluminum dissolving from the foil, the foil itself would have been pitted or at least dull in the areas affected.  The color for this reaction, however, is more gray-green than blue-green.

In both cases, the reaction may be accelerated by heat.

The mossy appearance may just be the way the food was described, or it may be areas where there was more or less marinade, creating a mottled appearance.  Finally, it may be that the protein in the yogurt, or in the chicken "goop" cooked enough to tighten, as described in Custards and Sauces, causing it to curdle.   For this to happen, however, the temperatures are such that the chicken would likely have had to end up overcooked and dry.  Combined with the discoloration, this might give the appearance described.  The yogurt is more likely to curdle if it was low fat.

As a final caution, if you are not sure whether something is safe to eat, it is advisable to follow the saying, "When in doubt, throw it out."  I would sooner lose a few dollars worth of food than take chances with  my health.