One pool guy told me that Black algae is almost impossible to kill. Is it true?

[bestinla]

Hi,
Our family purchased home with pool about 20,000 gallon.
So everything regarding pool is new to me.

I saw there were black thing bottom of pool.
So I called one pool guy, and he told me that it looks like black algae(he told me he is not sure. so He told to drain first).
And he told me that if it is black algae, then it is impossible to kill it because their root is deep.
he told me that we can only control. is it true?

I already drain water.
Please let me know what is the best way to kill black algae.
And BTW, how can I know for sure if is black algae or something else?

And not only black algae.
Please let me know what I need to do

BTW, is it better to hire pool guy, or is it simple enough(and not time consuming) to do myself?
In case I hire pool guy, what I can expect for him to do?

Please let me know.
Thanks.