According the Ed Naha novelisation of the film, the Ghostbusters have allowed Louis substantial access to their operation. He knows what Slimer looks like, that he's one of the earliest ghosts captured and some info about his habits of gluttony and mischief. At best guess, it seems likely that they've either provided a vivid verbal description or, more likely, allowed him to view various ghosts and ghouls that exist within their containment system:
You may also wish to note that in both novelisations, there's what appears to be a deleted scene set between the scene with Dana's bathtub and the scene with Dana and Venkman going out for dinner in which Louis attempts to catch Slimer after it escapes from the containment system:
Louis recognized the creature as one of the first trapped by the
Ghostbusters years earlier ... the Slimer.
Slimer, unaware of Louis's presence, glanced down-ward as Louis
glanced upward.
Both Slimer and Louis let out bloodcurdling yells and ran in opposite
directions. Slimer was the better for it. He disappeared through a
wall. Louis collided with the firehouse's brick wall and knocked
himself more senseless than usual. "Help!" he screamed to no one in
particular. "There's a thing"
Louis ran out of the room, knowing full well that Slimer would be back
for more food and that Louis had just lost at least three perfecdy
good Twinkies to an apparition.
and
The green ghost Slimer poked his head down through the ceiling and
scanned the room for the grub. "Okay, let's boogie," Louis whispered.
Louis whirled around and fired a proton stream at Slimer just as
Janine entered the room. Slimer retreated easily. Janine gulped and
ducked as a ragged bolt of energy streaked across the lab and seared
the wall behind her.
Louis stood there, trembling. "Ohmigod!" he shouted. "I'm sorry. I
didn't mean to do that. It was an accident."
He flip-flopped across the lab to Janine. The recep-tionist slowly
straightened up. "What are you doing up here?" she asked him.
Louis began to sweat. "I was trying to get that smelly green ghost.
The guys asked me to help out. I'm like the fifth Ghostbuster."
Stills from this scene can been seen in the Ghostbusters II: Storybook

and described in more detail (along with another still photo) in the Ghostbusters II: Junior Novel:
