I drank water out of this coconut and then left it unopen in a fridge for few days, and this happened - see the yellow film. It is sticky and the taste is neutral. Doesn't look like mold. Is it bad, or is it good natural process?
From the question and the comments, this looks a lot like bacterial film growing on the inside of the coconut near where the water was housed, that is the area with the most moisture availability for the biofilm to form.
Coconuts are meant to be quickly eaten and / or processed after opened, if you want to save the meat for later, open the coconut, take the meat, dry it out and preferably freeze it or if you are in a dry sunny place, you can sun dry it.
Leaving it opened in the fridge without any further processing is more or less playing russian roulette with nature, it is only about luck and only a matter of time until it gets bad.