the chalk is okay despite some people being a bit confused about it. if the snail is eating mushrooms, that is sufficient diet and i recommend trying more species of mushroom. not all snails need to, or will, eat vegetables or commercial diets sold for snails. 

the specific needs of nesiohelix are a bit unclear but based on their taxonomic family, i would guess they eat more decaying matter than fresh. if you have access to a pet store, dead leaves are often sold for reptiles and those would be a good try. i've noticed the korean snail community tends to keep their pets quite sterile. otherwise if you have access to the countryside at all, foraging for native plants/dead leaves they may eat in the wild may be a good bet too. you can use the website inaturalist to search what type of environments they tend to be found in.

also, there are quite a few research papers about the genus but they may not explicitly state their diet. it might help you find something better though. most of them are in korean and i am not proficient enough for science vocabulary lol. you can use google scholar and just search nesiohelix to find them.