Will it eventually say declined or something on the mempool?
No, it won't say anything. There is no singular mempool - every node has its own mempool. Some have dropped it already, some will drop it eventually, some may never drop it at all.
Or what exactly am I waiting to see if it is stuck and never gets confirmed?
It's already stuck. It's been stuck for three weeks. You should be contacting the sending and asking them to replace the transaction with one paying a higher fee.