I have never heard a cleaner shrimp's antennae turning brown, diatom bloom?
I don't know but my cleaner shrimp is lonely right now. Have a fish for him to clean, and he'll be happy. Since I don't have any fish right now, I hand-feed him two pieces of flakes per day
. Other than that if you keep your water parameters right, he'll be fine. Watch the temperature, one time my heater boiled my cleaner shrimp, he died so I got a new one.
Keep in mind that molting is not always a good thing. Shrimp or crabs often molt because they are stressed. Often when they are shipped, you'll receive them and noticed that they molted, it's because of stress. Other times however, they molt to grow back their antennae, after being attacked by mean fishes.
I hope that helps, probably the other might be able to help you with the brown antennae
.
stupid_naso