My husband and I purchased a 48 hp Evinrude a few months ago and have been doing some recent maintainence on the engine. We've noticed that when the motor starts (using a flusher of course...), large clouds of white smoke come from around the prop and lower unit. We've also noticed that after we shut off the motor, large deposits of gray sludge are collecting on the prop. The motor was running for 3-5 minutes and when we shut it off, the sludge was already pretty thick...

I know two-strokes burn oil, but I was wondering if the gray sludge collecting on the lower unit is normal and what this is coming from. The gas/oil mixture is relatively new. I've never owned an outboard before, so I just want to make sure nothing is wrong with the engine or lower unit

Your help is greatly appreciated! Thanks.
Compression is good on both cylinders. How can I test for a leak somewhere?

white smoke and grey sludge is not normal.
The white smoke could be a bad gasket letting some water into the power heads.
Grey sludge is new to me,,,I'd have to see it .Maybe its the lower unit's oil leakng out.
How does the motor run??
Run it some more and pull the plugs to see what they look like.

You have lower unit grease and water mixing with exhaust gases, creating the sludge described... a seal / gasket is bad in the lower unit.
It might cost a couple hundred but the boat shop mechanic is best for removing and replacing it... sorry.