Shella snail's secret specials are done with charge inputs.
The heart is done by holding away from the opponent (so like walking back, or crouching and holding away) for 1 second, the quickly tapping forwards + attack. You know if you charged for long enough if hearts start appearing around shella
the thunder is done similarly, but with down and up. so you hold down for 1 second, then quickly press up + attack
Important to note that you can charge during other attacks or movement. For example, shella snail can shoot a normal heart by just pressing attack, and while the animation is playing, she can charge heart charged heart.