If you open up a hotspot, you should build in a password so that only the ones that are allowed to use your hotspot. 

Android 9
Step 1:  Open the  settings
Step 2:  Tap on  Network & Internet
Step 3:  Tap on  Hotspot & tethering
Step 4:  Tap on  Wi-Fi hotspot
Step 5:  Tap on  Hotspot password
Step 6:  Choose a password and confirm with  OK
  1. Open the settings
  2. Tap on Network & Internet
  3. Tap on Hotspot & tethering
  4. Tap on Wi-Fi hotspot
  5. Tap on Hotspot password
  6. Choose a password and confirm with OK

Android Instructions
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