What is the Army uniform policy regarding Marine deployment patches?

Prior Marine here.....short answer, no. As a prior Marine, you can only wear the patch if you were a soldier attached to a Marine unit. Being prior and having deployed while in the corps is not enough to be able to wear it. Do people still do it, yeah, but it's wrong.

SPC Onel Cruz There is specific guidance from Army Human Resources Command that SSI-FWTS shoulder sleeve insignia for wartime service (erroneously called a deployment patch) earned in another branch is not authorized on the Army Uniform. Wear of USMC SSI FWTS is authorized only in specific enumerated circumstances by MILPER message (future) and AR670-1 (current).

AR670-1 a(2):
(2) Personnel who served in a designated area as a civilian or a member of another Service, but were not a member of the U.S. Army during one of the specified periods are not authorized to wear the SSI–FWTS.

You will need to transfer over your prior service badges and awards. https://www.hrc.army.mil/content/Sister%20Service%20Awards

It's for combat deployments. However, your deployment in the Marines will not be eligible for a deployment patch. That's because Marines don't award combat patches to Marines.

As long as it’s on your erb you can wear it bro

I get that AR 670-1 states, "Authorization to wear a SSI indicating FWTS applies only to Soldiers who are/were assigned to U.S. Army units that meet all of the criteria outlined in AR 670–1."

However, I have seen at two occasions General Officers authorize Marine Corps patches for Army Soldiers. The first was this one:

https://www.1stmardiv.marines.mil/News/News-Article-Display/Article/540712/soldiers-earn-right-to-divisions-blue-diamond/

The second time were actually former Marines in my Army guard unit who had been given permission by the state Adjutant General at the time to wear Marine Corps patches from units they served in combat with while still in the Marines provided they had documentation.

Were these official Exceptions-to-Policy (ETP), because it goes against the Regs? I don't know (I would assume so). But truthfully, who really cares outside some SGMs with way too much time on their hands (sorry to my friends who are SGMs, I love you guys). My personal feeling on it is that if you served with that unit legitimately in combat, I could care less what branch you were at the time and I won't call you out on it. But that's just me. The regs are there for anyone to read.

AR 670-1 Chapter 21-17 is what you are looking for

The technical answer is that unless you have the letter awarding you the right to wear it, you can't. Some Army units did receive the authorization to wear it from both 1 Mardiv and 2 Mardiv. That being said, only the idiots who get their panties in a knot will give you flak for wearing it. You deployed so wtf is the big issue? The ones who will give you flak would also be completely fine with saying soldiers who were in Kuwait and never left rightfully deserve one.