The expression is “is getting late” and it suggests the meaning of “it’s too late” or “it’s late enough that I should do something.” A guest at a party: It’s getting late. I better go. Or It’s late. I better go. The former is softer and less emphatic than the latter. ‘It’s getting late’ is often added when refusing a social invitation to make it more polite. A: Do you want to get a drink together? I know a great bar near here. B: Thanks, but it’s getting late. I’ve got to get up early for work tomorrow.(The same meaning as ‘No’ but softer and more polite.) Your expression in the future tense “will be getting late” is not as common but certainly possible.

“It’ll be getting late after that” or “it’ll be late after that” would be the correct way to use those phrases. “It’ll get late after that” doesn’t sound natural in my opinion.

Hi Mona, Do you have English Grammar in Use by Murphy? If yes, I will include unit numbers in future answers.

Using the verb to be can change the mood of the sentence. I would guess that the majority of native English speakers know it, though they may 'sense' it. http://guidetogrammar.org/grammar/to_be.htm#:~:text=In%20Passive%20Constructions,when%20the%20subject%20is%20unknown. Note the reference to passive writing. A fairly complex subject . Good luck!

Hi Mona, basically these two usages are almost the same. subject +will + be +ving usage sounds more formal and regarding my opinion, it is more advanced. Being + ving structure explains that the situation is ongoing I hope I could explain. Take care