I haven't seen brown before but I have had some eggs where there was a bluish grey blot on the egg from liquid in the eggs staining the shell. The stain was caused by decay inside the egg after the embryo died. If you have any liquid leeching through the shell and discoloring it I'd say the embryo died and there's decay occuring. You are at some risk of the egg exploding because a decaying egg is a rotton egg. Since you're ready for lockdown you can candle and look for movement or just check to see if the egg stays warm when you've had it out for a few minutes. If it cools off it died. I'd pitch it if candling doesn't show any movement (and it's hard to imagine it will). If you decide to keep incubating it I'd at least put it in something like a lunch sack or bakery bag so cleanup is easier if it does pop. Normally a rotten egg will have a funky cheese smell. But duck ang goose eggs have thicker shells and it's much less noticeable than it is with chicken eggs. Fortunately that means the egg spontaneously exploding is rarer too.
Also, your eggs should mostly be full of duckling at this point . You'll probably only be able to see anything at the end where the air cell is. The air cell should also not move. If what you're describing looks more like a lava lamp than a big dark mass filling most of the egg there probably isn't a duckling there.
