Landlord/tenant-neighbor

The property I rent has two other tenants, all owned by the same person.
Next door is a house and directly behind that house is a room. The room is supposed be for storage or a laundry room. But the landlord rents this room illegally. The occupant of this room recently built a storage room up against the original structure, about the same size of the original room. Now making his room twice as big and about two feet from my property. So now he has moved is lawn furniture and plants and junk, etc... over on to my property
about 10 feet and he thinks this is his property now.
In the past (before he moved in) I have put up a aboveground pool and the landlord was fine with it. So, this summer I ask the landlord if I could put up my pool and she said yes. I also told her that I found out that the city has a ordiance and there is a State Law, that any pool 3 feet of higher has to have a fence securing the pool. She said find out what the regulations are and if we need a permit. But don't get the city involved because she doesn't
need any problems with the city. (Because the structure the guy in the back built was not permited nor is it legal. He is not supposed to be living there. The room does not exsist with the L. A. County Assessers Office or with any Real Estate property searches.) . Basiclly, the landlord said I could not put up a fence because the guy in the back does not want us to take back our yard. So now I am violating the city ordiance and a State Law. I Have purchased a fence for $385.00 and then she tells me I can not put up the fence after she already said I could.
What can I do to get this guy out of my yard and what can I do about the landlord telling me one thing and now telling me something else because the guy in the back is throwing a fit? Also, one more thing, the Landlord and the other two tenants all speak spanish and we do not.
Please let me know what I can do if anything?

Do you have a written lease? Does the lease specifically define what the boundaries of your leased property are? If not, it may be hard to hold the landlord to any promises.
If you feel there are health issues, safety issues or zoning issues with this neighbor in the other room, you can file complaints with the city’s code enforcement division (planning). The names of the complainant are kept confidential, and the city will follow up to see if the landlord has proper permits for the buildings.