As far as I know, the unit does not cause terror, but does become immune to fear and terror causers. Also, because the Slann is in the middle of a TG unit, when someone wants to charge you, they do not have to make a Terror test as they are not coming in contact with the terror causing mode (in this case, the Slann).
In terms of combat resolution you only have a US3 model causing fear so it's doubtful you will auto-break anyone.
In terms of the 6" starting terror test, since you are a big block, most folks that close will already have charged you or be in combat, so this test is essentially useless.
I find the only value in a Slann causing terror is if your run him solo.