Salut,
Do you mean that the word does not exist in the Arabic language (although it appears in some dictionaries)?
I saw in Lane's lexicon this :
مَرْجَانٌ, a coll. gen. n.; n. un. with ة {مَرْجَانَةٌ}; (L
Small pearls: (AHeyth, T, Ṣ, Ḳ

or the
like thereof: or
large pearls: (El-Wáḥidee

or
coral, بُسَّذٌ, which is
a red gem: or
red beads; which is the meaning assigned to the word by Ibn-Mesʼood, and is agreeable with the common acceptation thereof; or, accord. to Et-Tarasoosee (or, as in the TA, Et-Turtooshee, and so correctly accord. to MF)
certain red roots that grow up in the sea, like the fingers of the hand: [vulgarly pronounced مُرْجَان:] the ن is said to be an augmentative letter, because there is no Arabic word of the measure فَعْلَالٌ, except such as are reduplicative, like خَلْخَالٌ: but Az says, I know not whether it be a triliteral-radical word or a quadriliteral: (Mṣb

IḲṭṭ asserts it to be of the measure فَعْلَالٌ. (TA.)
Lanes Lexicon