Is interrupting one's crying during prayer, out of fear that someone might see you, considered an act of arrogance towards the worship of Allah or an act of disbelief, or is it merely a whisper from Shaytan (waswasah)?

Not weeping does not constitute disbelief (kufr) or apostasy (riddah). Apostasy is severing Islam through intention, a statement of disbelief, or an action, whether said in jest, out of stubbornness, or out of conviction.

Concealing tears carries no blame and is not considered apostasy; rather, it is a form of waswas (whispers from Satan).

If the cessation of weeping is due to fear of showing off (riyaa'), this is not appropriate. For a Muslim's eagerness to conceal their good deeds is commendable and legislatively desirable. The Messenger of Allah, peace and blessings be upon him, said: "Whoever among you can have a hidden good deed, let him do so."

Sincerity should not lead you to abandon good deeds for fear of showing off, as this is a form of hidden riyaa'. Al-Fudayl ibn 'Iyad said: "Abandoning a deed for the sake of people is showing off (riyaa'), and doing a deed for the sake of people is polytheism (shirk), and sincerity is for Allah to protect you from both."