So your Selenite could have several different things going on with it. From a mild oxidation with a mineral within the crystal, meaning it might be 100 percent Selenite. To it is being fed a negative energy and needs to be thoroughly cleaned and cleansed.
It could also be that it old and it is changing colors, there are so many different reasons as to why it changes its color. Selenite actually comes in several different colors each one coming from different mineral contents to age.
Cleaning your Selenite:
You can run some warm water over it [Please note: do not soak it or fully submerge the crystal because too much water will dissolve it] and clean it off with a small soft brush- tooth brush, makeup brush, etc. If there is still dust and or particles stuck in your crystal you can take a piece of bamboo and pick/flick them out. Do not use a metal cleaning brush because you can end up scratching your crystal.
.If that does not work you can dilute some dish soap and rub it over the crystal, rinse and then brush it, again paying close attention to the amount of water that it is exposed to. Once you have cleaned it, set it out in the moonlight to charge it back up.
Other cleaning & Cleansing techniques:
Some also suggest putting it in a bowl of salt for several hours making sure that you cover it with the salt. I personally might be wary of this, as it might cause scratches on the crystal itself.
Other suggestions include burying it in the ground to cleanse it and bring it back to its full potential, but if you do this you might want to follow up with the cleaning techniques listed above.
While it is generally considered a self cleansing crystal it is always good to consider how to care and clean it. Selenite collects energy and it collects things in the air like incense, oils etc. All of which can make it change color, it is in the gypsum family so it is prone to environmental factors.
I hope this helps & Until next time ~ Blessed are we all! ~LunaOwl- J