Why are the highlights crushed and turned to grey in Photoshop?

[beahaxby]

I demonstrate the problem with two layers in the video. With the first layer, the highlight on my arm is turned grey. When I add the lava layer, it turns almost completely to grey. How do I get rid of this?

[c.pfaffenbichler · Accepted solution]

Is View > Gamut Warning checked? 

[c.pfaffenbichler]