It's very possible that the trooper, as a member of the 501st Legion which Kenobi fought alongside on many occasions, did recognise his former General but chose to keep it to himself. Likewise, it's possible that he had forgotten what Kenobi looked like, or maybe even didn't know.
The new Star Wars: Dawn of the Republic: Visual Guide has revealed that Temuera Morrison's veteran clone trooper from the Disney+ series Obi-Wan Kenobi is none other than Nax, a member of the 501st Legion.
Obi-Wan Kenobi couldn't possibly know this, however, and it's still remarkable he showed compassion by giving Nax credits. Nax, for his part, may well have recognized the Jedi General he once served alongside. Nax's fate seems symbolic of the entire clone army.
Because of where Obi-Wan Kenobi falls on the Star Wars timeline (nine years Before the Battle of Yavin), it's possible that this could be Captain Rex of the 501st Legion. The bearded and beaten clone looks similar to how Rex appears in Star Wars Rebels, which takes place just four years later (5 BBY).
It's very possible that the trooper, as a member of the 501st Legion which Kenobi fought alongside on many occasions, did recognise his former General but chose to keep it to himself.
Why Palpatine REFUSED to Confront Obi-Wan Directly (Was He Scared?) - Star Wars Explained
Did any Jedi wear armor?
During the Great Galactic War and the Cold War in the years following the Treaty of Coruscant, many Jedi Knights donned parts or even entire suits of heavy armor when going into battle.
After a decade of abuse from the Empire, civilians, and perhaps the process of overcoming brainwashing resulted in the veteran clone trooper's failure to see that it was Obi-Wan Kenobi who gave him credits.
Whether or not he was the rebel commando named Nik Sant is the subject of debate: However, in the New Canon Rex did serve with Luke, Han and Leia at least once.
Starting off with a familiar face here from episode two. While Obi-Wan is on the neon-soaked planet of Daiyu, he comes across a beggar on the streets. But this is no ordinary beggar, it's a veteran clone, played by none other than Temeura Morrison, the star of 'The Book of Boba Fett'.
In the Disney+ series "Obi-Wan Kenobi," Kumail Nanjiani got to pretend to be a Jedi in a world in which they actually exist. Nanjiani played the role of Haja Estree, a con man who pretends to be a Jedi, using tricks and subterfuge to convince people that he is one with the Force in exchange for credits.
In the original trilogy he is introduced as Ben Kenobi, an alias he uses while in hiding from the Empire. He is a mentor to Luke Skywalker, to whom he introduces the ways of the Jedi.
While living in a homeless condition on Daiyu, Nax wore dirty Phase II clone trooper armor adorned with the blue markings of the 501st Legion and was in possession of an accompanying helmet, which he used as a bucket to collect credits. His arms were bandaged, and he wore a green poncho.
Captain-turned-Commander Rex was a leading member of the 332nd. As part of the 501st Legion, the 332nd Company fell under the command of Jedi General Anakin Skywalker and Clone Captain Rex, who had both previously been joined in command by Jedi Commander Ahsoka Tano before she departed from the Jedi Order.
Rex is actually 31 years old, though his biological state is likely closer to at least sixty years, due to the accelerated aging that was genetically installed in all Clone Troopers. Because of their accelerating aging, Clones age twice as faster than regular humans.
Rex was 100 percent loyal to Anakin and was his truest friend out in the field. Anakin had to hide so much of himself from other Jedi, but he could be completely open with Rex, and he knew that Rex would never betray him.
Since Obi-Wan and Anakin often fought together, Obi-Wan was friends with R2-D2 as well. This means that although R2-D2 might not have immediately recognized Obi-Wan on Tatooine due to his age, Obi-Wan should have recognized the droid.
The sequence calls back to a similar scene in Revenge of the Sith, when Bail Organa orders Threepio's memory be erased to prevent him from sharing information about Anakin Skywalker's fall to the dark side.
However, there were some Jedi that wore Jedi robes in other colors. Vandar Tokare and Zhar Lestin, for instance, would wear blue robes around the time of the Jedi Civil War, while their contemporary Vrook Lamar wore red robes.
During her time with the resistance, former Jedi Padawan Ahsoka Tano was given a Mandalorian-designed combat outfit with beskar plates. At some point in the Clone Wars, the Sith assassin Ochi of Bestoon had a deathblade made of beskar.
