leak at temp sensor??

[geospider]

discovered this today.
Once up to temp and pressure,found a leak near the temp sensor. I replaced this since it didn;t work the PO had a bunch of plastic type "stuff" around the base of it that I had to remove to get old one out and new one in.
Now I see that was some kind of sealer that they also had on the top of the head toward the back of motor.
I am hoping I need to tighten the sensor, but I don't imagine they need to be very tight.
there is a certain leak: see pic in the area. can't tell if crack in top of head or at sensor. seems a strange place for a crack.
http://imgur.com/V7o0N7j

geo