If you notice any pink or brown discoloration on the top, that means your product has been contaminated with bad bacteria, and you have to toss the yogurt. Do not eat if this happens.
If you want really thick coconut yogurt, refrigerate your coconut milk overnight and then in the morning (do not shake). Open and scrape off the top layer and place that in the bowl. Do not use the watery portion of the can. You can save the watery part for soups, stews or smoothies.