Niebla is a noun in Spanish (fog). We literally say in Spanish "There is fog", but the translation in English uses a different structure because saying "There is fog" would sound very odd. With the most natural translation "it's foggy" in English you use an adjective (foggy). Bear in mind that with these weather expressions some of them translate in a slightly different way.
Niebla is a noun hence hay niebla
You could say está neblinoso using the adjective
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