it looks like you have both bryopsis and cyano going on there...
do you have a bunch of detritus underneath your rocks?
do you run GFO and GAC?
what do you feed?
what do you have for flow?
and of course, what kind of skimmer/filtration do you have?
I would use a small media reactor and start by using carbon with some sort of phosphate remover. Try to remove all the smaller rocks in the back of the tank and siphon out the detritus unless you need them there. Also increase flow as suggested and maybe siphon all the cynao on the sand. Manual removal also works good as long as you eliminate the source.
awww come on now, your going to scare the guy....lol
bryopsis CAN be beaten, IS beaten, and WILL be beaten...
I would say the issue can be attacked twofold...
first, it sounds like you need to take out all your live rock to clean out the detritus hanging out. do it hafl a tank at a time so that the fishies have a place to hide...
but I would keep like 3 buckets of water from a water change, (dont siphon detritus for this WC) and then take out each rock individually and scrub the bryopsis off with an extremely firm plastic brush (try home despot in the painting section, for paint removal, should be right next to the steel brushes).
then rinse the rocks in succession in the other buckets to get off any remaining algae/detritus. then of course, clean all the detritus off the sandbed. then after you get everything put back together, you can attack the water parameters that are allowing the byopsis to acheive it's evil foothold in your system.
second, this is where using GAC and GFO, as well as rinsing all brine from frozen foods and stopping use of most dry foods starts...
then the obvious, increased wc's with ro/di (like one every 2 weeks),
increase your mag to 1500 with that pathetic kent product that only serves this purpose in life....lol (otherwise you should never use bottom of the barrel products like kent) and maintain your ph and alk at the higher end of the acceptable spectrum. DO NOT make these chemical changes quickly, instead take several days/weeks to increase these levels slowly.
then as mentioned, you probably need more flow if that much detritus is collecting around your rocks in the first place.... there should be enough flow to keep detritus in suspension in the water column so that the corals can eat it and the filtration can then remove it.
which of course brings me to the inevitable "what kind of skimmer/filtration do you have in your sump"
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