I was actually running into the opposite problem, my tank was so clean my chaeto was melting out. But I do have some of the mini nass, a couple hermits, and a few cerith/nerite down below to help!Maybe you could add some of the hardier seagrasses (ex. Halodule wrightii) to help with detritus and nutrient accumulation in the substrate alongside the Mangroves.
http://www.reefkeeping.com/issues/2006-04/sl/
https://aquaticusplants.com/
Or you could use Nassarius snails… I guess.
I’ll have to check that sea grass out