Depends on the source of the phosphates. It may be due to overfeeding, not using RO/DI water, accumulation of detritus on the sand bed, etc. It's always best to directly address your source.
In the meantime, however, a phosphate reactor (Two Little Fishies or Bulk Reef Supply's Dual Reactors) would work well. I wouldn't put chaeto in the main display, in case my fish or other livestock nip at it.