Or you can just throw it away and buy a new one. They are rather inexpensive.
OP is in SG, I doubt even a CE4 or 5 is inexpensive there.
I think it's just some residue from the juice on the wicks. All you need to do is as others have suggested. Wait until you are ready to refill it, then instead of refilling with e-liquid, give it a good rinse out with hot running water. Shake off the excess and refill with e-liquid. Just make sure you dry the connector well where it screws into the battery.
When you first clean it, you will notice it be a little light on the vaper it produces, but it should be fine again after just a few puffs.