I just got a new 72gal with a lot of live sand out of a pre existing tank. I left the sand in some buckets over night, and when I came back in the morning, there was a carpet of red on top of the sand. So I'm having the issue. I'm pretty sure their just bristleworms, and they've had some time to reproduce in the system. Personally, I wouldn't worry about it, for bristleworms will eat the detritus in your tank!