Arw you exposed to sunlight frequently and for long periods of time?
If it’s just a brown spot, the most probably it’s a simple mole and there’s no need to worry about it. Some get born with them, while pthers get them from sun or different circumstances in their lives. In most cases they are harmless as they’re considered to be simply a benign tumor. Be sure not to mistake it with a malign one, the benign tumos are the ones that don’t spread, while the malign ones are the cancerous tumors