Stuck in amphitrite city

Hello, i’m french and sometimes I don’t fully understand the dialogs ^^' I talked to the amphitrite champion and he talked about someone to see in a cave in the lake in the south of the city, i’ve been there and found nothing but a white aura or something weird at the end of the cave (is this something for the late game ?). I tried to go to the gym but it’s closed saying “gone fishing” and the mount rose access is blocked so… what do i do ? thank you

you have to use mew and use the tesseract abillity on the white thing when your next to it, then go around and use it again from the other side to go to the other side.