As stated it is difficult to say for sure without a picture but...
1. Is the yellow area on the stalk or on the cap?
If it is on the stalk and the stalk is not deteriorating then IME/O nothing to worry about.
If the spots? are on the cap then I suspect they are caused by a build up of detritus. Any time dirt, leftover food or fish waste etc. lands on the cap of the leather, it should almost immediately be blown off by current in your tank. Small amounts are cleaned from the cap when a leather sheds, but heavier deposits that are not blown off can create a "sore spot" that often turns yellow, especially on thick capped, long polyped leather varieties (almost like putting something solid and heavy on grass then when you lift it up a few days later the grass under it has yellowed but will come back with light and water
)
2. "If" this seems to be the case then adjust the flow in your tank (or add some) so that the leather is recieving enough current to keep it clean.
3. It may not be opening because it is stressed due to the "sore spots". With good current and a clean cap it should heal and be back to opening normally very quickly (1-5 days)
FWIW
SiF