What is any energy resource that can be used in place of fossil fuels called

I think those are usually called "alternative fuels", or
"alternative energy sources".  Those are things like
wind, solar, hydro, geothermal, tidal, nuclear etc.

I don't like that description at all.  It makes it sound as if
there's something about them that's not quite right, and
that we don't really take them seriously.

The only thing that makes them "alternative" is that we're
just now discovering them, for the first time since we left
the caves. 

Up until now, we've always been happy to burn the stuff
that's cheap and easy to get from a hole in the ground. 
But we're just now starting to notice that it's poisoning
the air we breathe, and plus, so many people have been
using it for so long that now, it's becoming harder to get. 

So we're starting to look around now and see what else
we can use to do the job.