Lemon grass needs a LOT of nutrients. A LOT. Seed starter has nothing, it is just a clean medium to start seeds in. Get that plant some rich dirt and a good nitrogen fertilizer. That should help. Lemongrass also has very very deep roots. Or, at least, it wants to. If it was dug up it is probably completely freaked out at this point and needs to regrow feet of roots. Can you get it some 'root grow' food?