There are multiple things that could cause that, so it's best to go through everything so as to pinpoint the likely issue and resolve it as fast as possible rather than just tell you to turn down your light. Fill in the gaps, if you will:
What are you growing in (DWC, RDWC, coco, etc) feed and watering habits?
pH?
EC?
Temp and humidity of canopy?
PPFD of canopy (or if you don't know that, the brand and model of light and hanging height and dimmed setting)?