Rebuilt engine issue?

[The Swed]

I noticed a small leak on my heater hose on my intake and it sprayed on my carburetor (see attach picture). What causes me a great deal of concern is notice how thick the coolant is and it looks like some kind of sealant to me. I had the engine rebuilt by a local shop. It has 300 miles on the engine and I'm to return it at 500 miles to change the oil to make sure nothing came loose etc... Is this a new practice to put sealant like that in a engine because we didn't do that back in the 80s. Is this to reveal a leak if one occurs? Sorry the picture is sideways and I don't know how to fix it.



Thanks for your inputs
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