Roof condition

I currently has no sign of leak, but couple water drop from the TV antenna pulley in the last few days. So I climbed up to the roof for a quick look and saw these (please see the pictures).

I saw on internet, usually people use silicone to seal the roof.  Does mine looks normal? Or the previous owner use something else instead of Silicone to fix?

Does it mean the seal need to be replaced? If yes, how should I remove the old seal? 

Please suggest. Thanks.