SMD LED: How to calculate optical power (in mW) from luminous intensity (in mcd)?
Okay, let me try to solve this problem.

Parameters: peak wavelength 527 nm, Luminous Intensity 0.18 cd at 5 mA and 2.9 V, emission angle 120 &deg;.

From Wikipedia article "Candela" :


  The candela is the luminous intensity, in a given direction, of a
  source that emits monochromatic radiation of frequency 540×10^12 hertz
  and that has a radiant intensity in that direction of 1/683  watt per
  steradian.


So 1cd = 1/683 W/sr = 1.464 mW per steradian.

Luminous intensity measures (lm and cd) are linked to human eye sensitivity curve, which complicates re-calculations into optical power. The maximum in light frequency in the above definition corresponds to wavelength of 555 nm, which is pretty close to the green LED emission, so we can ignore the difference along the "Photopic Spectral Luminous Efficiency Curve". 

The LED emits into 120&deg;, but if we want to calculate total emission, we need to integrate over all angles, so the "effective" (ballpark estimate) angle for "flat emission" will be about +- 50&deg;. One steradian is +-33&deg;, so the led illuminates an area of about (50/33)^2 = ~2.3 sr. The narrower is viewing angle, the brighter is LED appearance, but the emitted power is obviously the same.

Therefore, the emitted power is 1.464 mW/sr * 0.18 cd = 0.263 mW/sr, times 2.3 sr, or about 0.6 mW total.

The consumed power is 5 mA * 2.9 V = 14.5 mW, and therefore "luminous efficacy" of this LED is about 4%.

Corrections and comments are welcome.

POINT TO TAKE HOME: If you need a 6 mW radiated green LED, you should search for a 1000 mcd rated LED with viewing angle of around 30&deg;.