Could just be a slow release fertilizer added to the soil to help blooming. If not, they could be millybug nests (if it looks like cottoncandy it's most likely it). But as you seen no bugs, i don't think this is the problem. Yellow leaves : very common on Hibiscus, maybe just a lack of watering, sudden lighting condition change may cause leave to turn yellow as well. P.S. for a constant blooming, use tomatoe fertilizer ;)