These are 水柿, a kind of persimmon, or kaki (柿子). It is related to, but different from 紅柿, which is more common in Hong Kong these days. 紅柿 is very astringent (澀) until it is fully ripe. When 紅柿 is ripe, it is very soft and sweet. 水柿 is also astringent due to the presence of tanin. But the tanin is neutralized by putting it in lime water (石灰水). That is why there is a layer of white powder on its skin. It does look ugly and moldy, perhaps that is part of the reason why it is not popular these days.
However, wash away the white powder and peel it. And you will be rewarded with the beautifully symmetric, firm, crisp, sweet, orange color “flesh”. The good ones are just as good as those Japanese persimmons that are popular these days. But at a fraction of the price. Typically you can get 8 to 10 水柿 for 10 HK dollars. Whereas you probably have to pay 10 times or more for a real Japanese persimmon.
It should be obvious 水柿 is one of my favourites. Unfortunately, they are available only for a few weeks each year, which is right this moment.