You are not right. The Hydra gains two heads back for every one head cut off. Plus, this riddle is not like the riddle of the Sphinx, where morning, midday and night are proverbial terms.
It's a snowflake, but the riddle's description is not good. It should be "six arms and no legs" instead of "six legs and no arms". And "swarm" should be "cover, blanket" etc. "Swarm" implies a roving horde, but snowflakes fall gently.
I run, though I have no legs to be seen. I possess no heat, yet I do have steam. I have no voice to let words out, but from far away, you can hear me shout.