Legal Question in Real Estate Law in California

You can ask your neighbor to remove it. Without knowing all the facts and details I can't say if you have a legal right to get a court to order it removed. You could also possibly negotiate a lot-line adjustment with your neighbor to put it on his side of the line and give you an equal amount of property elsewhere on his side of the line to even it out. That is a very common solution to encroachments like this.

The neighbor may have an easement for the septic tank. Such an easement could arise from a deed condition granting or reserving the easement. This is likely if your parcel and the neighbor's parcel were once a single, larger parcel that was subdivided. An easement for the tank and perhaps its associated piping (the sewer line and the leach field) could also arise by prescription......five years of being there without permission...... but prescriptive easements also have an "open and notorious use" requirement, and it may be difficult for the neighbor to establish that this condition was met. (There are cases holding that underground water pipes were open and notorious because "they had to be somewhere" even though they couldn't be seen, but I'm undertain this would apply to a septic system). A pleasant negotiation might be the neighborly thing to try, but in order to be bargaining with a full hand, have a local real-estate lawyer check out and advise you on the easement situation, both as to recorded and prescriptive easement possibilities, beforehand.