Maleza translates to "weeds" or "undergrowth." It's an interesting word because it's not commonly found in song lyrics, especially in a metaphorical sense.
In "Tristeza Maleza," Manu Chao pairs it with "tristeza" (sadness), creating a powerful image of sadness as something wild, pervasive, and perhaps even suffocating, like overgrown weeds. This unique combination gives the word a deeper, more poetic meaning within the song.