arcturians can't be anything but humans, and as mentioned above, probably something like thai ladyboy stuff.
after all, when the colonial marines wake up from hypersleep, they mention about ripley
"apperently, she saw an alien once"
to which another respond:
"well whoopdief*ckingdoo, im impressed"
they do mention about 'bughunts',
so they probably have seen some big bugs, obviously on other planets.
i guess it's safe to say they never encountered any aliens for real, any non-human species up untill then.
during the board meeting after all, the ECA rep says the following:
"To be perfectly frank, we've surveyed over three hundred worlds and no one's ever reported a creature which, using
your words.....'gestates in a living human host' and has 'concentrated molecular acid for blood.'"
in the full script, they mention at LV-426 they did mention lifeforms but it was a virus (which is interesting alltogether).
one has to assume thus that they're not encountered an alien species, to be honest.
the portrayal of the story now thanks to prometheus and covenant will make that board full of it, as we can be sure that
weyland-yutani sent the vessel delibaretely to that direction, and put ash alongside to retrieve alien specimen.
they were aware of the existance, which is probably why it's so interesting to them - they have simply never ran into aliens.
