Each balance meter in Nexus has five stages. Let us assume that stage 1 is heavy tipping od darkness and stage 5 heavy tipping of light. Stage 3 is actual balance, and that's what Nexus wants.

1 - Nexus is ally of Fort as the two cabals have the same goal of reducing the darkness. Nexus might raid evil cabals and hunts evils.

2 - Nexus is ally of Fort, but only provides retrieval assistance. Does not necessarily hunt evils, but does deny them any aid.

3 - Balance. Nexus doesn't meddle in other cabals' affairs.

4 - Nexus denies goodies aid and helps evils to retrieve or in other ways. Does not necessarily hunt goodies and doesn't go for the orb.

5 - Nexus might hunt goodies and potentially raids for the orb.

In stage 5, Nexus may be hostile enough to warrant raiding. In this stage, it is viable to strike Nexus in retaliation if the state of the Balance hasn't changed to 4 or below. The exact when depends of the current crop of Nexus and how aggressively they react to different states of balance.

What you must understand here is that Nexus is not evil, and therefore Nexans are never hunted due to Fort objective to "kill evils". However, sometimes Fort objective to "Protect the Light" may require hurting Nexans or possibly even striking the Nexus itself if the situation gets bad enough. This is, when people who aren't inherently anti-Nexus and view killing Nexans as a bad, regrettable thing, but do so because they think it is lesser evil than letting them run rampant in that situation.

What comes to Nexans attacking you when Light holds sway, you should discuss the attack with Nexans and try to talk things out. You should also find out the state of balance (4 or 5) during each individual attack. After turning the other cheek for a couple of times during that state of balance, you can attack that particular Nexan during that particular state of balance. However, you should also acknowledge that Nexan A might behave differently as function of Balance than Nexan B, meaning that the pattern you've observed only applies to that particular Nexan. Thus, witnessing Nexan B repeatedly attack light during balance state 4 doesn't mean that you should attack Nexan A during balance state 4.

If you're "being Pro" about it, to use Raybaer's wording, and demand that either you can attack Nexans or you can't attack, no caveats, then the answer is that you can't attack Nexans. Attacking Nexans is an exception, not a rule.