IF....IF you are going to leave grey tank valve open then make a dip in you sewer hose so critters/sewer flies etc can not come back up and set up house in your grey tank (dip holds water -----'P' trap)
Course leaving it open allows crud to build up because the water trickles out leaving the solids behind (hair, skin cells, body oils, food debris, grease/oils, soap scum, etc)
Grey stuff accumulates, dries out, rots, stinks etc etc.
So no wonder you have 'critters'
Good strong dose of bleach (1/4 cup per 15 gallons to sanitize-----and then a FULL TANK 'whoosh' drain to remove 'stuff'