Fluoride-induced dental fluorosis is called "ash-teeth." why does this happen?

Too much, too young: Too much systemic (swallowed) Fluoride prior to age 6 (while the permanent teeth are forming under the gums) disrupts the tooth forming process. Once the tooth erupts into your mouth fluorosis can not occur and small amounts benefit the tooth enamel. The key is taking advantage of fluorides topical benefit and avoiding its systemic side effects.

To much of a good th: Volcanoes erupting magma, gases, ash. Grazing animals eat grasses in this Fluoride enriched environment: fluorosis, hence the term ash-teeth. Too much of a good thing causes the teeth to be discolored. However, the fluorosis is only cosmetic. These teeth are extremely resistant to cavities!

Dental Fluorosis: Fluorosis occurs from systemic ingestion of tooth much fluoride during tooth development (meaning before they erupt). Fluorosis can be treated cosmetically with minimally invasive acid etching/whitening for less severe cases to bonding/porcelain veneers for more extreme cases.