Flail Snail and Invisible Stalker are both valid choices for conjure elemental
Choose an area of air, earth, fire, or water that fills a 10-foot cube within range. An elemental of challenge rating 5 or lower appropriate to the area you chose appears ...
Firstly, the Flail Snail is CR 3 so it is a valid choice for this spell. The Invisible Stalker is CR 6, which would be invalid, but the spell can be upcast to increase the CR range, so if the spell is cast with a 6th level spell slot or higher, the Invisible Stalker then becomes a valid choice too.
From the PHB, pg. 225:
At Higher Levels. When you cast this spell using a spell slot of 6th level or higher, the challenge rating increases by 1 for each slot level above 5th.
Secondly, both of these creatures are "elementals" (which is one of the creature types like "humanoid" or "beast"); this is what the spell is referring to when it says:
An elemental of challenge rating 5 or lower
not just those that are called "Elemental" such as "Fire Elemental", etc.
This creature type is outlined in the Monster Manual, pg. 6:
Elementals are creatures native to the elemental planes. Some creatures of this type are little more than animate masses of their respective elements, including the creatures simply called elementals. Others have biological forms infused with elemental energy. The races of genies, including djinn and efreet, form the most important civilizations on the elemental planes. Other elemental creatures include azers, invisible stalkers, and water weirds.
Also, both of the creatures you mentioned are each associated with one of the four elements. Flail Snails are associated with earth (Volo's Guide to Monsters, pg. 144):
A flail snail is a creature of elemental earth ...
And Invisible Stalkers are associated with air (Monster Manual, pg. 192):
An invisible stalker is an air elemental ...