"The last weapon" (1953) by Robert Sheckley, available at the Internet Archive.
Two (?) adventurers/archaelogists are inside of the weapon cache left by Martian civilization.
Edsel was in a murderous mood. He, Parke, and Faxon had spent
three weeks in this part of the deadlands, breaking into every mound
they came across, not finding anything, and moving on to the next.
The swift Martian summer was passing and each day became a little
colder. Each day Edsel’s nerves, uncertain at the best of times, had
frayed a little more. Little Faxon was cheerful, dreaming of all the
money they would make when they found the weapons, and Parke
plodded silently along, apparently made of iron, not saying a word
unless he was spoken to.
Eventually they find a chamber marked "Absolute weapon" (or ultimate weapon).
At the end of the room was a door. There were letters in Martian
script engraved on it.
“What’s that stuff say?” Edsel asked.
“Something about ‘final weapons,’” Parke told him, squinting at
the delicate tracery. “A warning to stay out.” He opened the door.
Both men started to step inside, then recoiled suddenly.
Note that a second mention of said weapons is made later on, this time with the titular "last weapon":
“With those troops — the force-field — and the weapons — I can take
over the world.” He said it coldly, knowing it was a fact. He turned
to go back to the cave, to activate the Synthetics, but stopped first
to pick up the little black box Edsel had carried.
Engraved on it, in flowing Martian script, was, “The Last Weapon.”
I wonder what it could be, Parke asked himself. He had let Edsel live
long enough to try out all the others; no use chancing a misfire
himself. It was too bad he hadn’t lived long enough to try this one,
He opens the lock, releasing sentient blob that eats his fallen companions and then him, as well.
Well, he told himself, let’s see what the Martians considered their
last weapon. He opened the box.
A vapor drifted out, and Parke threw the box from him, thinking
about poison gas.
The vapour mounted, drifted haphazardly for a while, then glimmered
white in the dying light, and Parke saw that it was just a tremendous
mouth, topped by a pair of unblinking eyes.
“Ho ho,” the mouth said. “Protoplasm!” It drifted to the body of
Edsel. Parke lifted the blaster and took careful aim.
“Quiet protoplasm,” the thing said, nuzzling Edsel’s body. “I like
quiet protoplasm.” It took down the body in a single gulp.
A protoplasm, if I am to believe Wikipedia, is "the living content of a cell that is surrounded by a plasma membrane".
I found it by CTRL + Fing this page for
weapon. It's a page containing a short description of all Sheckley's stories, which I had found when answering Short story by Sheckley about a man from the future who radiates love and lust.