That there is a sky with clouds moving around in it. Allah's miracle:
He created the heavens without pillars that you see and has cast into the earth firmly set mountains, lest it should shift with you, and dispersed therein from every creature. And We sent down rain from the sky and made grow therein [plants] of every noble kind. -- Qur'an 31:10
Do you not see that Allah drives clouds? Then He brings them together, then He makes them into a mass, and you see the rain emerge from within it. And He sends down from the sky, mountains [of clouds] within which is hail, and He strikes with it whom He wills and averts it from whom He wills. The flash of its lightening almost takes away the eyesight. -- Qur'an 24:43
That you're alive and have the ability to see the sky. Allah's miracle:
And He is the one who gave you life; then He causes you to die and then will [again] give you life. Indeed, mankind is ungrateful. -- Qur'an 22:66
And Allah has extracted you from the wombs of your mothers not knowing a thing, and He made for you hearing and vision and intellect that perhaps you would be grateful. -- Qur'an 16:78
That some clusters of water vapour in the sky approximately resemble the short word Allah (الله)...? I do not think it's befitting to attribute something so meagre as one of God's miracles.
It's also risky to value these things highly, because clouds also randomly form non-Islamic shapes, e.g. a crucifix. Patterns will also randomly appear in mediums that would not be considered appropriate (e.g. birthmarks on pigs, a splattering of urine on the bathroom floor).
If you look at enough data you will find patterns, regardless of whether or not the patterns are meaningful. How many clouds does one have to inspect before discovering one that resembled Allah in Arabic? In science, we use statistical significance testing to filter out what could happen just by chance. Perceiving familiar patterns in randomness is called pareidolia in psychology.
Basically, it's best not to take these things too serious, saying "oh, that's cute" and move on.