Moon's negative greenhouse effect? Using the standard formula for effective temperature of a planet, Venus has T(eff)=220K, Earth has T(eff)=255K and Mars has T(eff)=212K (using albedo for Venus a=0.7, Earth a=0.367, Mars a=0.25). Average surface temperatures of 730K, 288K and 218K respectively are easily explained by differences in greenhouse effect. The moon however with an albedo of a=0.1 should have a T(eff) of about 270K, but the average surface temperature is about 250K. 
Here is where I got the T(eff) formula and here is where i got the average temperature of the moon. (the latter is a new citing and I have edited the mean temperature value) I have revised my planet temperatures using this reference.
What explains this negative difference?
You don't say how you got your average temperature. If it is a straightforward average, then I think I have your answer.

The temperature of the moon is highly non uniform, ranging from around 100 K on the night side to 370 K on the sunlit side. This gives a crude average of 235 K.

https://space.stackexchange.com/questions/3290/what-is-the-temperature-variation-between-the-lunar-day-and-night-on-the-moon

However, in terms of blackbody radiation, if we treat both hemispheres as separate blackbodies, then together they radiate as much energy as a hotter blackbody because of the $T^4$ factor in Stefan's law. The "effective temperature" will be 
$$ T \simeq (0.5\times 100^4 + 0.5\times 370^4)^{0.25}= 311\ K$$
So this is more like your equilibrium temperature and a more appropriate area weighting and/or tweaks to the exact range of temperatures could bring it into close agreement.