I was having a discussion the other night with my gaming group, about whether or not an enemy mage could see my character while astrally projecting. He was wearing a rythium polymer cloak, which combined with the fact that he was hiding meant the mudanes running around the area couldn't spot him easily. However, apparently he was 'seen' by one of the earth elementals.

Now, I'm not sure if its a matter of the GM picking on my character for various reasons, but just how easily is someone spotted from the astral plane?

All life has a glowing aura visible on the astral plane. The table for Astral Patrolling is in MitS, but it's not exceptionally hard to spot someone from the astral if they are not hiding (other than the suit).

Really he's going to see you plan as day if the only thing hiding you was the Ruthenium. Ruthenium does absolutely nothing to hinder vision on the astral when worn like a predator suit, and elementals permanently see the astral.

If all you were doing was wearing the ruthenium suit and standing still, then you would be in plain sight to anything on the astral plain, because you're aura would still be in plain view. Things get a little harder if you were activly trying to hide, since getting behind a large, solid object can shield you from astral view just as easily as from physical view.