Lump
A dirt demon is just a lump. Just like a dust bunny is just a clump of dust.
Picture a lump of dirt. Is there a demon hiding under there? No. It's just a saying, ya nerd.
かい
かたまり
None
Have you ever heard of the game "Katamari Damacy?" That game is about a little guy who rolls a ball around, turning it into a bigger and bigger lump. What do you think the Katamari (かたまり) part means? It means lump, of course.
If you don't know the game and have never heard the music (especially), it's great stuff! If you have, this word will be really easy for you. If you haven't, look it up!