Transvag ultrasound @ 6wk3d shows good heartbeat but gestational sac measures smaller (1.6cm) and is cloudy. what could this mean?

Ultrasound: Depending on the type and sensitivity of the machine, everything may be normal. Since the heart beat was normal you aren't having any symptoms and all else is normal, i would recommend monitoring your symptoms to see if there is a change. Things will be significantly different in 1 -2 weeks.

Heartbeat: Seeing the heartbeat greatly proves the odds that pregnancy will not miscarry. I haven't heard of someone saying the sac was 'cloudy' before. Sac size --inform differs from your LMP -- is okay. Just means you ovulated at different time. I would ask for another ultrasound in 2-3 weeks if you're worried, but also know that good heartbeat means a lot.