In Julius Casear, why do Artemidorus and the soothsayer feel that Caesar is harmless?
I think you've got a bit muddled here. Artemidorus and the soothsayer both think that Caesar is going to be harmed, rather than "harmless" - I don't think either of these characters make any sort of comment on Caesar's on potential strength or dangerousness.
The soothsayer really only tells Caesar to "beware the ides of March" - we don't know anything else about his character, where he's come from, or whether anyone else knows who he is. That's up to the director in production.
Here's the text of Artemidorus' letter:
Caesar, beware of Brutus; take heed of Cassius; come not near Casca; have an eye to Cinna; trust not Trebonius; mark it well Metellus Cimber; Decius Brutus loves thee not; thou hast wronged Caius Ligarius. There is but one mind in all these men, and it is bent against Caesar. If thou beest not immortal, look about you. Security gives way to conspiracy. The mighty gods defend thee! Thy lover, Artemidorus.
I've emboldened the line that's of interest to your question: and what does it mean, but "if you're not immortal, keep your eyes open". So Artemidorus knows that Caesar is human and also suspects that there is a conspiracy against him.
So do they think he's harmless? No. Do they think he's going to be harmed? Yes.