Aliens can only do signature attacks. Some of the signature attacks are a special kind of grapple. An alien grapple follow special rules as listed in the specific signature attack, but unless specified otherwise, it is still a grapple. The "special" with an alien grapple is the effect of the grapple, not that it is a grapple.
I’ll take it. Would you allow other PCs to help with someone who is grappled by a xeno? Using one of their actions in combat of course.
If a PC is grappled by a stalker (or scout or drone) with result #4 (ready to kill), then any friendly character that has an action before the stalker's next initiative can help the poor fellow to break free and of course the grappled PC himself also must have an action to spend before the stalker's next initiative.
When it comes to the facehugger's grapple attack (#4), they do a grapple and if we read the description the grapple in order to have an effect needs to be in effect over the PCs turn (especially the arm tangle (3-4)), but the signature attack doesn't list a base dice for the break grapple action, like it does for the stalker. Here either assume that the facehugger is to weak so any success with the break grapple action will suffice or use 6 base dice for the facehugger (as in signature attack #5). If anyone stands next to the happy couple, they can help to break them free, if they get the time to do it before the facehugger's next initiative.
I haven't read all signature attacks in the book, but the other signature attacks I have seen have effects that happen immediately and doesn't involve grappling over several rounds.