the flat wroms are the ones on the gsp pic right? i only see it clearly there... i was under the impression they are bad for mushrooms? i had some a while back, and they were just all over my mushrooms, i treated the rocks my mush rooms were on, and no more flat worms... i only had to do one treat ment. it was super easy... just used one of those kids plastic aquariums as a quarentine. i know if you treat your display, it gets kinda crazy with pollutants, dead flat worms... so i guess i just spot treated them... and alot died off... FWE works so well! i used the salifert kinda, i do'nt know if there are other brands...
Those flatworms look like Planaria flatworms. I have had an infestation in my nano this last year. They are not harmful to corals, but will multiply until they literally cover and smother other corals to the point of depriving them of light and oxygen. WetWebMedia has some excellent information on their flatworm faq pages and here:
Flatworm Exit is probably your best bet, just remember to run carbon and do large water changes afterwards to ensure any unwanted chemicals (the flatworms release some noxious chemicals when they die) are removed.
Best of luck with the sale (your xenia are way cool )