Is it 'safe' to use 200ohm resistor to reduce 9v to 5v
https://www.amazon.com/ask/questions/Tx1M6VEO0CSRRC2/?source=allQuestionsPage

The seller says these are self regulating which means it will draw 20mA at 5V. What confuses me is the seller says it draws 30mA at 3V where these lasers are diodes thus they should always have a fairly fixed voltage drop that only changes when current changes. By this logic the diode would be brighter at 3V than at 5V. Anyways we can deduce that:

V = IR
R = V/I
R = 5/0.02
R = 250Ω


Looking like a simple device, it should be that a 250Ω resistor is already connected in series with the laser diode. If you want to power it from 9V then:

V = IR
R = V/I
R = (9-5)/0.02
R = 4/0.02
R = 200Ω


You would need to add a 200Ω resistor in series. So you're correct.