Not personally, but I've read stories and it's been known to happen. Ethiopian opal is, to put it simply, porous and absorbent. It will soak up everything - water, soap, body oils, perfume, creams, lotions - if it's liquid and comes into contact with it, the opal will absorb it. The result often is discoloration, so if you've been wearing it on a ring, perhaps, something like this could be the cause.
I used to like Ethiopian opal, but don’t plan to go for long outings with it, or use the restroom, because at some point you’ll have to wash your hands with soap, and even that is a no no with these stones. Perhaps keep it as a pendant stone to avoid lotions, soaps, sprays.
There’s a reason they are much less expensive than their Australian counterparts.
I bought an Ethiopian opal 8 months ago, the body was a greyish white, and it is slowly becoming yellow. Any similar experience?