Yup, that's not your snail, it's your snail's future children. As jake37 says, you will have many baby snails unless you remove it. That may sound like fun but unless you want to start breeding them you'll have to deal with water quality changes and a lot of poop. To remove, just take a paper towel and scoop it out.
Tip for future egg sacs (and there will be more):
Each morning look into the tank and look up, around the top and lid. Snails are nocturnal, they lay eggs at night, above the water line. One of mine actually laid eggs up at the filter above the lid once, then went back inside. They can lay as often as twice weekly.
You can tell how old they are by how mushy the sac is: totally mushy, they were laid last night. Slight crust on the outside, it was 2-3 nights ago. Harder crust, they've been there a few more days. They leave less of a mess and don't totally mush out if you give them 24 hours before removing.
They hatch in 2-3 weeks if left alone. You'll see the sac becoming whiter with dark cores - those are the snailettes (my word) forming.
What fish do you have in there? Some like to eat those eggs, therefore problem solved. But if no one is dining on them and you don't want a snail farm, wipe 'em away.