On the lower unit of the outboard housing, right at the waterline, my 150 merc motor has developed white stains that I cant remove.

I looks like algae, but it isnt.

Have any other merc owners had this happen and any advice for cleaners, etc?

Have you been running this in brackish water?

If it is hard water lime deposits, try a little Lime Away.  I use it on the motor when mine gets that way.