you have to create an entity for it too.
go to entity bank\generic\decals
and you'll see that each of the decals in the
gamecore\decals folder, also has an entity in the entity bank\generic\decals.
you must make a new decal entity that references the decal in the decal bank.
I would advise copying the lava or slime decal's FPE and using that, because it is already a decal that goes on the floor.
hope that helps!
"It's often better to keep your mouth shut and let people think you're an idiot, than to open it and confirm their suspisions" - Noel Coward