Learn Spanish (Latin America): Hay algo extraño aquí.

Yes! Just like how adjectives usually come after nouns in Spanish, they also come right after words like algo (something) and nada (nothing). Because algo does not refer to a specific male or female object, the adjective always takes the default masculine singular form. That is why it is extraño and not extraña.

How would I say 'There is nothing strange here'?

You would use the negative word nada (nothing) instead of algo. The sentence becomes No hay nada extraño aquí. Notice that Spanish requires a double negative here, so you must put no before the verb and nada after it.

Why do we use hay here instead of a form of ser or estar?

Hay is a special word used specifically to say that something exists ('there is' or 'there are'). You use forms of estar (like está) to talk about where a specific thing is located, but to simply announce that 'something' exists in a place, you always use hay.

Could I say Algo es extraño aquí instead?

Yes, that literally means 'Something is strange here.' However, Hay algo extraño aquí ('There is something strange here') is the much more natural and common way Spanish speakers express this thought.