Assuming you are talking about CN, the Beach Grill is the only restaurant open after 10p every night, but you have Otaheite, Cassava Terrace, and Heliconia open for dinner on Mondays. Lychee is closed for the Repeaters Dinner on Monday nights only.
Monday is the beach party. The food is amazing and they put on a spectacular show while you eat on the beach. By far my favorite night. Monday night is also the repeaters dinner in 8 Rivers and that is only open to repeat guests. So technically, yes the pool grill is the only restaurant open on Monday night but there is a plethora of food!