Meaning of "coral cover"

[Sweetpumpkin]

Hello,

I have two questions on the sentence below:

The coral cover has diminished by 50 percent in the past 10 years.

1) What is coral cover? Is it the area the coral is distributed?
2) Is it okay to put 'last' instead of 'past' there?

[Mister Micawber]

1-- Yes, the covering of the sea floor by coral, measured in square meters/kilometers.
2-- Yes
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Native English teacher at Mister Micawber's