im starting to think its a long tentacle plate too. and yes they should be placed on the substrate. id say besides the close quarters with fish, and that urchin, NICE TANK! a lot farther along than i was at 3 months.
I also never liked a background. I have a 75 fish only and a 29 gal reef that is geting to full. I picked up a used 55 gal with stand and no light fo $50 found a great buy on live rock $70 for 80lbs and a 260 watt pc light for $100 anyway it the 55 gal tank back is painted a blue I am thinking that is going to look sick but man after puting in the water sand and rock it looks great Now I want to do somthing to the 75 maybe find a solid blue that could be taped up back there.