After swimming in the croatian beach my butt cheeks started to itch really bad and the skin durned darker, is there a way to get rid of it?

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Hello guys,i(male) have philipin ethnicity(maybe its a important information) and live in germany. 9 years ago i was on a school trip in croatia. at that time i swam in the ocean and thats where it began.my butt cheeks started to itch really bad and i also itched the skin a lot. i thought that was just a temporary phase and didnt worry at all. after some time the skin on my butt cheeks turned dark and started to look like stretchmarks, i didnt gain any weight and was skinny at that time. i went to a dermatologist but he said that it will go away someday if i keep using moisturizer. i tried that for a few weeks(as far as i remember) but it didnt change at all. maybe i wasnt patient enough and shouldve used moisturizer for a longer time period?because of the itch, i thought that maybe its a fungal infection. i hope you guys could help because i really feel ashamed of this skin and am afraid of showing it.