Fire shrimp along with cleaners and other members of the shrimp family are more active during the night as they are semi noctournal. So if your shrimp stay hidden within the rockwork throughout the day and then come out at night, no alarm as this is perfectly normal. As mentioned above, feeding will eventually bring them out and about for a short period of each the day.
I did a night dive in Mexico once and was amazed by the army of shrimps I saw patrolling the sea floor
