It could be a case of you adding too much food waste to the tank for it to handle at such an early stage (even though you do have a lot of live rock), this could be the case especially if you are not using another mechanical filter other than the skimmer. The decaying shrimp could be giving you the ammonia reading.
I would suggest stop feeding the tank..... The corals do not NEED food, feeding them aids in growth but honestly I would wait until your tank is aged a big longer before you start feeding corals, especially meaty foods like mysis.
The hermit crab in there will find enough food off the rock.
Stop all feeding for now, and do a few small water changes (5%) over the next few days to a week, and see if the ammonia reading goes down.
-Diana