Just append :after to your #alertlist li:hover selector the same way you do with your #alertlist li.selected selector:
:hover can easily be appended to :after or :before in the just the same way it would to any otehr element.
li:after {
color: red;
}
li:after:hover {
color: blue;
}
or in SCSS
&:hover{
&::after{
color: blue;
}
}
More here.