The rule is that you can't attack an orb while a neutral sslak guardian is still alive in the zone with the orb, regardless of whether or not the sslak is currently guarding. In addition to that, the neutral sslak guardian receives a guard marker each upkeep phase. Reference page 8 of the Domination Rule Book. This guard marker bestows the same benefits on the neutral sslak as it does any other creature, meaning it prevents you from attacking other objects within the zone and it provides the creature's quick attacks with the counterstrike trait. If your opponent also had a guard in the zone, you would be free to choose which guard to attack.
I can't find it in any of the rule books other than the Academy book, which doesn't really apply to Domination, but I remember talking with Laddinfance about guards only protecting friendly objects. That way in a free for all game, you could ignore one opponent's guards if you were attacking a different opponent and people couldn't team up and protect each other, or accidentally protect each other when they just wanted to defend themselves. This rule seems to have made it into Academy, and even though I can't find it anywhere for Arena currently, I would recommend using it in Arena as well.
If you do follow that rule, then neutral sslak's on guard do not ever protect your opponent's creatures, and your opponent's guards do not ever protect a neutral sslak. If there was only the neutral guard, you could choose to either attack it or to attack any opposing creatures in the zone. If there was only an opposing guard (the neutral sslak having already been hit once this round) then you could either attack the guard or attack the neutral sslak. You could not attack any of your opponent's other creatures though, since they have a friendly guard. If there was both a neutral and an opposing guard, then you would still be able to choose which to attack as mentioned previously.
I hope this helps.