When a coral expels "brown stuff" its usually one of two things. The first of course actually is waste, but the second is zooxanthellae.
This is the symbiotic algae that lives in the coral's flesh, what soaks up light and gives the coral any extra energy it has, and also what lends the coral its color.
When an animal with zooxanthallae is stressed it can expel the symbiotic algae the same way it wastes and you may notice bleaching.
I would try putting the coral in a lower light area if possible or if not add a few layers of screen or other transparent material to slowly acclimate it to your lights.
Check at night to see if it opens its feeder tentacles. You may offer it mysis and if it takes it and eats it thats a healthy sign.
Most LPS corals can do great in higher light but need to be started in lower light. These tend to get more puffy.
Let me know if you notice any bleaching, your parameters seem perfect, so my guess is its the lights. The thing to consider with this is how your lights are different from store lights as well as where it was harvested in nature.
Hopefully this info helps. It could be many other things but this would be my educated guess
