Moved to sick fish/coral forum:
It doesn't bode well for the stars future if missing an arm. Depending on how it was damaged it may not survive. Does the online site where you bought it from offer a guaranty on stock for any time period? The one arm shouldn't foul your large tank so it's worth the risk IMO
to add it. Cleaning your glass shouldn't be an issue.
With a 150 gal
tank (hopefully with tons of base/lr
) the star might make it but I'm worried your system still might be too new and could still be prone to parameter swings since you set this up 4 months ago.
If you haven't read it already I'd take a look at INVERTEBRATE NON-COLUMN by ROB TOONEN
which is one of the best Linckia articles on the net.