If you are using fresh corn, cook it, remove the kernels and reserve.
Dice the tomato, onion and chop up the cilantro leaves.
Squeeze the limes.
Clean the mussels. The best way to clean mussels is to first soak them in fresh water for about 20 minutes. Next you want to cut off the byssal threads or more commonly known as the "beard". An easy way is to grab the beard with a paper towel and pull it hard toward the hinge end of the mussel. You never want to pull it toward the opening end or death to the mussel will occur. An even easier way is to cut the beard off with some kitchen scissors.
If there is any crud on the outside shell, you can remove it with a kitchen brush. And very important, the mussels should be tightly closed. If you find one that isn't, throw it out. It is most likely dead and not worth the chance of getting sick from.
Combine the corn, tomato, onion, cilantro, lime juice, oil and hot sauce. Adjust the seasoning with the salt and pepper.
To cook the mussels - In a large soup or stock pot, add the wine and bring it to a boil. Add the cleaned mussels, cover with a tight fitting lid and cook for a few minutes. The mussels are done when they pop open. You may have to stir them once or twice to get them to pop. If you find one or two that didn't open, throw it out. If you find a whole bunch that didn't open, give your fishmonger some grief.
You are going to serve the mussels on the half shell so you can toss the top half away. The mussels are attached to the shell so using a kitchen knife "shuck" the mussels. This just means to slide your knife along the shell to separate the mussel from it. Be careful not to cut yourself.
Leave the shucked mussels right on the shell. The reason for shucking them is to make it easier for the person to eat them. You don't have to shuck them and that would be fine. It just means the person eating them has to separate meat from the shell with their teeth. Actually, no big deal.
Top each mussel with the tomato corn salsa and arrange them on a serving plate or individual plates if this is a sit down dinner. It's nice to garnish the plate with some cilantro leaves and maybe some lime wedges.
That's it. A really simple but elegant appetizer.