soil question??

I've got some spots on the top of my soil in one of my pots??? Anyone know what it might be?? It doesn't seem to be affecting the plants growth or anything, it is the unhealthier looking of the two, but it's by no means on it's way out. I was thinking if anything maybe mold of fungus?? My humidity has never been above 55% but is normally and almost always at 50%. I found one thripe on the side of one of my pots, but I've already picked up some neem oil concentrate to take them out as soon as the plant get a lil more mature (it's almost three weeks into veg). Anyway, any help or idea's would be appreciated. Btw it's a bright lime green and it's in spots on different parts of the soil. In two weeks I've only had to water twice I'm using Miracle Gro Organic choice potting mix (should have got the potting soil, but to late now). 50/50 mix of perlite too.
thanks

-GT