I have seen many Pearl rays do this for no apparent reason. Sorry I don't have anything to offer other than flying saucer has said regards PH.
I have know Pearls to fully recover though as they get bigger and to this day I now have a mature adult that used to do this a lot as a youngster.
Food wise, try live earth worms. If it doesn't accept these then there is something wrong.
Also consider prazi or flukesolve.
What is you temp at?
And switch off lights for a couple of days and remove the bass if you are not sure if this is a cause or not.