Hangul syllable nalp

It is a Hangul syllable, not a complete message. The label “nalp” describes its parts: initial ㄴ, vowel ㅏ and compound final ㄿ. It identifies how the syllable is assembled, not a word's translation; the surrounding Korean word gives it a role.

Is 낦 an emoji? No. In a message it works as Korean writing: it contributes a syllable to a word rather than a pictorial reaction.

Why are Hangul letters arranged in blocks?

The block format was designed to write Korean speech. Hangul's development is dated to 1443; the Hunminjeongeum manuscript, which explained the new alphabet, appeared in 1446. Before Hangul, people wrote Korean with Chinese characters and adapted methods such as idu and hyangch'al, which used those characters to record Korean sounds and grammar.

Hangul groups letters into syllable blocks, but its shapes were not directly copied from Chinese ideographs. The script later moved from manuscripts into print, typewriters and software input while keeping that block principle. South Korea generally calls it Hangul and North Korea Choseongul.

In digital text, the same syllable can be stored as one precomposed character or as a sequence of conjoining jamo, the individual Hangul letters. A font shapes that sequence into a syllable block.

If someone sends 낦 alone, what should I do?

Ask for the surrounding word or sentence. A lone block gives you one Korean syllable, not enough context to know what message the sender intended. Inside Korean text, read the whole word and sentence.