Welcome to R2R! If there's a giant plume emanating from your rocks due to something spawning, I'd recommend grabbing your family and running away from your house before the Kraken emerges! Maybe an expert or super long-timer knows more than me about this, but I doubt it's a spawning event. My first guess is that your rocks had a bunch of sediment in it that loosened up sitting in your tank water for a while and then further loosened up when you cleaned them. When you put them back in, the water flooded into the rocks and blew the sediment loose. Anyway, that's my guess---and I wouldn't be surprised if you saw a nutrient spike (NO3 and PO4) after a few days. One way or the other, keep monitoring the health of your inhabitants and check ammonia a couple times a day just in case the "plume" starts overloading your biofilter. If you do see an ammonia spike, do a partial water change right away and treat with Prime.
Regarding bristleworms, you'll find differing opinions about them but I think the majority of R2Rers (is that what we call ourselves?) would say they are beneficial clean up crew members (as long as they're regular bristleworms and not fireworms or, heaven forbid, bobbit worms). HTH and have fun!
