Mana Masses are formed through a process known as Mana Rebirth. It's when a mage suffers from mana exhaustion, though the Mana surrounding the mage flows into his or her's Pool. This happens because the Mana Pool for the mage somehow had a leak, allowing Mana to flow out, yet when the Pools do have an opening, it's rare for the Mana to flow back in. When they've become a Mana Mass, their body mutates, depending on the mage's personality, their old body destroyed.
They aren't more powerful, yet less powerful than other races. Though they do have one Strength and Weakness that other races do not have : They aren't able to die from Mana Exhaustion, but they can be absorbed by Magic users, making the Mage have a larger Mana Pool. This is due to the fact that they're living Mana. However, it's extremely hard to absorb them, as they have a life essence, so only powerful magicians can do it. The weaker the Mana Mass, the easier it is to absorb. The stronger the Mana Mass, the larger the Mage's Mana Pool is.
The only way Mana Masses can die is by being absorbed, then being used up by a mage. If their physical form is destroyed, they can use the mana around them, along with matter or energy, to reform their bodies.