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The book is Science Fiction Terror Tales, a 1955 anthology edited by Groff Conklin. It has appeared under several covers; the one with the red cover is the 1969 Pocket Books edition:

"The Leech", a short story by Robert Sheckley, available at Project Gutenberg and the Internet Archive.

"Arena", a novelette by Fredric Brown; also the answer to several old questions, such as Human & Alien captured, observed by higher beings; first published in Astounding Science Fiction, June 1944, available at the Internet Archive. A very famous story.

"Lost Memory", a short story by Peter Phillips, also the answer to the old question Robots on planet with no humans until a rocket with a human lands; first published in Galaxy Science Fiction, May 1952, available at the Internet Archive.

The book is Science Fiction Terror Tales, a 1955 anthology edited by Groff Conklin. It has appeared under several covers; the one with the red cover is the 1969 Pocket Books edition:

"The Leech", a short story by Robert Sheckley, available at Project Gutenberg and the Internet Archive.

"Arena", a novelette by Fredric Brown; also the answer to several old questions, such as Human & Alien captured, observed by higher beings; first published in Astounding Science Fiction, June 1944, available at the Internet Archive. A very famous story.

"Lost Memory", a short story by Peter Phillips, also the answer to the old question Robots on planet with no humans until a rocket with a human lands; first published in Galaxy Science Fiction, May 1952, available at the Internet Archive.

I remember 3 stories. One about a blob that lands on Earth and absorbs matter exponentially. They launch it into space and blow it up, and now infinite pieces will land on more planets.

"Arena", a novelette by Fredric Brown, first published in Astounding Science Fiction, June 1944, available at the Internet Archive. A very famous story. From the Wikipedia plot summary:

"Lost Memory", a short story by Peter Phillips, also the answer to the old question Robots on planet with no humans until a rocket with a human lands; first published in Galaxy Science Fiction, May 1952, available at the Internet Archive. ISFDB synopsis:

 
I remember 3 stories. One about a blob that lands on Earth and absorbs matter exponentially. They launch it into space and blow it up, and now infinite pieces will land on more planets.

"Arena", a novelette by Fredric Brown; also the answer to several old questions, such as Human & Alien captured, observed by higher beings; first published in Astounding Science Fiction, June 1944, available at the Internet Archive. A very famous story. 

From the Wikipedia plot summary:

"Lost Memory", a short story by Peter Phillips, also the answer to the old question Robots on planet with no humans until a rocket with a human lands; first published in Galaxy Science Fiction, May 1952, available at the Internet Archive. 

ISFDB synopsis:

"Lost Memory", a short story by Peter Phillips, first published in Galaxy Science Fiction, May 1952, available at the Internet Archive. ISFDB synopsis:

"Lost Memory", a short story by Peter Phillips, also the answer to the old question Robots on planet with no humans until a rocket with a human lands; first published in Galaxy Science Fiction, May 1952, available at the Internet Archive. ISFDB synopsis: