Shepherd of the Planets by Alan Mattox.
The ship is forced to land when their fuel pack fails:
Captain Renner looked from face to face.
"We were lucky to set down safely," he said to them all. "We might have been caught too far out for a landing. It is night now, and I am going to get some rest. Tomorrow we will see what kind of a world this is."
He left the control room, and went down the corridor toward his quarters. The others watched him go. None of them made a move to leave their seats.
"What about the fuel pack?" David asked.
"Just what he said," Farrow answered him. "It's exhausted. Done for! We can run auxiliary equipment for a long time to come, but no more star drive."
As you say, they find the natives who live on the lichen:
David brought in the one new point that was of interest. He had been out hunting among the boulders again, and it was almost dark when he returned. He told Renner about it at the supper table, with the others listening in.
"I think the natives eat the lichen," he said.
"I haven't seen much else they could eat," Beeson muttered.
"There's more of the lichen than you might think," David said, "if you know where to look for it. But, even at that, there isn't very much. The thing is, it looks like it's been cropped. It's never touched if the plants are small, or half grown, or very nearly ready. But just as soon as a patch is fully mature, it is stripped bare, and there never seems to be any of it dropped, or left behind, or wasted."
"If that's all they have to live on," Thorne said, "they have it pretty thin!"
The story proceeds as you remembered. The closing scene is (David is the linguist):
"David!" he said.
"Sir?" David asked, stepping forward.
"You understand their language now, don't you?" Renner asked.
"Yes, sir," David said.
"Then translate!" Renner ordered. "Out loud, please, so that the others may hear!"
"Tolava--" the natives chanted, bowing.
"Tolava--our father," David said, following the chant. Suddenly he swallowed, and hesitated for a moment. Then he straightened himself, and went sturdily on. "Tolava--our father--who art from the heavens--give us--this day--our bread!"
The story isn't quite as you remember it. The botanists on the ship work out they can make the soil fertile but to do this they need to harvest all the lichen to make compost, and this leaves nothing for the natives to eat. So they decide to feed the natives with synthetic food from their synthetizer while they are processing the lichen. Captain Renner doesn't deliberately play God. It's that a cargo cult develops among the natives.