Hirsyam Jamali

I cant seem to pinpoint the cause of this brown substance...is it a disease? Should i 3quarantine it?

Hi, newbie reefer here. currently having a nano setup with a few livestocks namely, 1 Reef lobster, 2 maroon clown and one open brain trach. I have been going by a friend's advise for the most part, everything seems to be nominal except for my pair of recent clowns who refuse to eat. Already acclimatised but when fed mysis,they won't chase after the food. Its been 3 days since I have them inside the tank, can any expert here advise? Thanks.