In my opinion, I don't quite buy the "tank too new" explaination. While it's true that generally new tanks take some time to stabilize, there are plenty of mitigating factors at play that can offset it. Keeping the bio load low, not introducing too many animals early, prepping your rock before hand to keep it from leeching, etc. are all factors that can contribute to keeping a new tank stable. For new people, telling them to wait and that a tank is too new for a nem is solid advice, but it's not the end all rule.
A few things that might help:
"Ritteri" anemones have been reclassified as h. Magnifica. They are classified as a difficult species for a few reasons. They are as follows:
1: they, like carpet nems, are colonized by opportunistic bacterias which are typically either E. coli, klebsiella, enterobacter, and some gram positive organisms like Staph, or a combination of the above. The nem needs these bacteria to fight off more deadly diseases in the environment. Without these bacterias, the nem would succumb to a more deadly pathogen and die. The bacteria is part of the animals "normal flora" in the same way it is present in the human gut.
The stress of collection stresses the animal and lowers its immune system. With the immune system not operating at peak function, these normal flora bacteria start to reproduce and colony counts begin to reach a point where the bacteria starts to infect body systems they normally wouldn't inhabit. Left unchecked, these bacterias will essentially cause a form of septic shock in the anemone and kill it.
Fortunately almost all of these bacteria are sensitive to floroquinolones and more specifically ciproflaxin is the drug of choice. The animal must be put in a capable QT system and treated with a fluoroquinolone in order to knock these bacterias out before they destroy the nem and will allow the animals immune system to return to peak function.
2: the brown stuff you saw being expelled was a symbiotic dinoflagellate within the anemone called zooxanthelle. Zooxanthelle is what is responsible for providing the animal with energy via photosynthesis. When the nem gets stressed, it will expel the zoo in an attempt at regulating its energy needs. Typically the nem will keep a small amount of zoo inside it, but it's not nearly enough to let the animal get any energy. So to make up the difference, the animal will need feed, heavily.
After the animal has been fed for some time, it's color will start to return as the zoo starts to tick back up to appropriate colony counts, and the feedings can stop as the animal will now be able to regulate it's energy needs with the zoo via the light.
3: nems are pretty sensitive to nitrates. Mags are especially so. Nitrates need to be kept very low to keep the animal from being poisoned. Have you checked them recently? With a decent test kit?
What's likely happened, is what typically happens with these animals. The stress of collection and moving lowered its immune system, the opportunistic bacteria started to multiply to unmanageable numbers. As the animal was over run with bacteria, it began expelling it's zooxanthelle and it could not have its energy demands met. All its energy needs now are being directed toward fighting the infection off, and the demand is so great that no amount of food can meet it. It can't digest the food anyway, since there isn't any energy available to complete the task. Without energy, and loaded with bacteria that are destroying its cells, the animals primary functions cease and it dies.
Hope that helps.
my SPG is at 1.025 temp stays between 76-79, PH is at 8.2, Nitrite is at 0 ammonia is at 0, nitrate is at 5ppm any advice would be appreciated my tank is about 6 months old. 
