would this alien be theoretically possible?

Alien.....

There are a couple of key points to your question. The first is that it is made of silicon. Gene Roddenberry explored that with the Horta. But it existed in an oxygen environment. (Clearly, because Kirk and Bones didn't have their own supplies of oxygen.) Everyone seems ok with that since silicon forms multi-atom molecules, as does carbon. (Both are in group 14.) The second is will chlorine support this alien life-form? We know that there are anaerobic life-forms near hydrothermal vents in the ocean that are supported by hydrogen sulfide (H2S). So will chlorine serve a similar function as oxygen? Like carbon and oxygen, and the fact that carbon is oxidized by oxygen, silicon is oxidized by chlorine gas, forming SiCl4. A xenobiologist needs to chime in here.

A quick Google search, one without qualifying the results, turned up this: http://www.xenology.info/Papers/Xenobiology.htm

When I was on the planet Zork we had creatures like that.

Yes. There are bacteria here who lives in extremely hot thermal vents so it’s possible for creatures to live in hot climate.

It really is suspected that there is some form of living thing like that in volcanoes and beneath the earths surface, in the molten layers. They are allergic to the atmosphere as it is right now.