gray spots on my seedling, not good! please help!

there has always been a tiny gray spot toward the end of the left leaf of the biggest pair of leaves. now it has expanded and the same spot has appeared on the right leaf. What could this be? its grayish is it some kind of mold? I fear it will spread and kill it, please help its my only baby

cover up the rest of the cube with more pebbs.feed 1/3 strenth put your nutes in a spray bottel and keep spraying 3 times a day untill it has a good root system this will help him out....good luck hope its a female.........feed with a 20.20.20.npk1/3steanth untill shes bigger tomato feed will do................

but what is it?

its a deficiency of some sort. I cant remember...phosphorus maybe...but dont take that as gospel...def deficiency though.