"Phazon Leeches thrive on the bioenergy drained from weakened and dead prey. They often roam in groups, leaving behind toxic trails as they slide across terrain in search of sustenance. Their bodies are soft and unprotected, making them easy targets to dispatch."
The fact that these creatures are small, unimportant life forms, and that they feed on corpses makes them similar to Sbugs in Super Metroid. However, unlike Sbugs, Phazon Leeches do cause damage if they come in contact with Samus, and can be killed.