i-adjective
easily smudged; prone to bleeding; easy to spread
Describes ink, makeup, or other substances that tend to run, bleed, or spread easily. Often used for pens, eyeliners, or stamps.
このペンはにじみやすいから、薄い紙には使わないほうがいい。
This pen smudges easily, so you'd better not use it on thin paper.
夏は汗でアイライナーがにじみやすい。
In summer, eyeliner tends to smudge because of sweat.
にじむ is the base verb meaning 'to blur' or 'to run (of ink)'; にじみやすい is the adjective form meaning 'easily smudged' or 'prone to bleeding'.
Compound of 滲み (continuative form of 滲む 'to blur, run') + やすい ('easy to'). The kanji 滲 is not a jōyō kanji, so the word is usually written in kana.