You dont need metal halides to keep nems or corals, they can keep kept under t5's just fine. You just need a specific amount of light. The quick and dirty rule is to take the total number of watts and divide by the number of gallons you have.
So for the MH set up, you have 1140 watts/180 gals = 6.3 watts per gallon, which is perfect for keeping nems (around 6 watts/gal is what you shoot for with nems).
But the T5's have a total of 260 watts/ 55 gals = 4.7 watts per gallon. This is actually fine for any coral you would want to keep, but on the low side for nems. So i wouldnt keep anything more demanding than a bubble tip. I clearly did my math wrong the first time, im sorry for that.
the general rule of thumb is 3 watts per gallon for corals, and 6 watts per gallons for the more light demanding animals.
But that only applies to metal halides and t5 lights with individual reflectors for each bulb. Power compact lights or t5's that dont have individual reflectors need much more higher watts per gallon to keep the same animals because they quality of light (power compacts) or total amount of light (t5 without individual reflectors) is much less.
LED moon lights are totally for your pleasure they dont serve any needed function.
If you want to do LEDs as you main light source, im not really sure what the rules of thumb are, but i would go with the one above