WIBTAH if I start doing the same thing back to him?

Definitely would not be the AH.
Speak to your husband IMMEDIATELY about this if you haven't! Why is he filming you in the first place? This is beyond weird and boundry crossing!! I'm so sorry that you're dealing with this.

If you can, there's some cheap, but functional indoor security cameras on Amazon. They're small enough to be hidden/kept out of sight. They can even be mounted on the wall. You might be able to catch him this way.
It's not ideal, but I'd make it appear that I was recording whenever he was around. Even if it was just me with my phone "taking selfies' or "hey guys so today.."
Truly hope this gets resolved for you.

Tell him off the second he films you.
Have your partner stand up for you.
This shit is mental

He is not a good person to have in your life.
He's the nutcase himself.

The things he's doing to you can cause you to get unwell mentally. Or PTSD. But I know you will never let that happen.

I so hope you are able to move away soon.❤️

MTFOOTH…. Tell your partner you got to move.. that’s creepy

I would try to move out ASAP

This is creepy AF and also illegal. Filming someone in a private space without their consent is a legal issue and the higher the expectation of privacy is in a given space (e.g. a home) the more illegal it is. He has no leg to stand on at all.

Maybe is better to just leave this feeling “I want to catch him” just go you will be fine
GO!

Theres a very high chance he is filming you in private too e.g. shower. Move out asap and get your husband to cuss him out.

Time to move

He sounds extremely perverted! And I don't know what state you're in, but in most states, if a person is recording someone else and the person being recorded says, "I do not consent to this." the person doing the recording can get into legal trouble. Personally what I'd do next time he is blatantly recording you is firstly say "I did not consent to this." then pull out your phone and record him back, but of course you have to make sure it's heard that you're asking for his consent to be recorded. Why on earth has your partner not confronted him about this!?

Yeah, like everyone else is saying, move if possible! Also, I repeat, please fill your husband in on what's going on. Someone else said that he may have cameras that you're not aware of (like the shower!), and this really tripped me out. It's definitely time to move.

Can the police do anything? You know for harassment or something They would have evidence on his phone when police take it.

Honestly. Run from this place and if your husband thinks it's normal leave him behind. You can't be safe with insane people. That's wild

i assume only three people live in your house. bring it up when you all three are together. like casually ask him why he does that and put him on spot.

If you live in the same house leave, I know that's not always easy but if you can i highly recommend you do and go no contact. I would also look into the legality of it, if it's illegal I'd be making an official police report. I'd also not be allowing my child around him. If you haven't mentioned this to your husband yet Do So.

I didn't stop to consider that you were outside of the States, my apologies there. I'd look up the laws in your country, even more specifically, your hometown or "state", and see what the laws are governing being recorded without your permission. Have you filled your husband in on what's going on?