Yes and that is why you dont walk downstreet flaring in the astral with all your foci active, they draw attention and the bad kind of attention.
5ed, unlike the previous editions, wasnt able to explain that properly, but that kind of strategy is part of the setting, send someone on the astral plane to be the spotter, then cast some mana spell using a ritual with a few other guys to nuke that enemy.
They cannot, however, be targeted unless they are actively casting a spell or using a bound active focus.
Examples:
A) You cannot astrally attack a mage that has no spells or foci active.
B) You cannot attack someone using a focus katana if he isnt the person bound to it, as they get no benefit from it other than it being a sharp katana.
C) Complementing #B, you cant attack the person bound to the same katana, as simply being carried by someone doesnt make it "activated", even if someone else is using it in combat.
D) You could even ready an action while following someone to cast a spell as soon as that person flares in the astral plane.
E) People on the astral can also attack you while you are astrally perceiving, because you will also flare in the astral while looking at it.
PS: That also works for your enemies, they can do all kind of nasty stuff to you. So, never, EVER, leave mana trails behind. Takes only a few minutes to remove your signature from an area where you cast your spells, and will save you from ages of headaches.
Now to your other question:
Yes it can be targeted, but follow the normal rules for attacking objects. If you destroy the object, the foci is destroyed aswell, as their astral-physical connection is severed.
For general astral rules: page 312-315 5ed core rulebook.
For foci and astral auras: page 318 5ed core rulebook.
About Physical vs Mana Spells and Astral beings: page 282 5ed core rulebook.
About how to fight astral enemies: page 361 5ed core rulebook.
All those references are further explored on magical sourcebooks from previous editions.
