If nothing has changed in the last week (like lights, flow, etc..) then itÃ"šÃ‚´s called skin shedding.
They do this to get rid of algae, waste, detritus, etc...
ItÃ"šÃ‚´s normal unless it does not open for a long period of time.
Either it is that or not, be sure to check the behavior of the angels, and check water params.