Learn Croatian: Kada razina mora bude rasla, izgubit ćemo obalu. - When the sea level rises, we will lose the coast.

Because clauses starting with kada (when) or ako (if) that refer to a future event usually require Future II. You form it with the auxiliary verb bude and the past participle (rasla). It shows that this action provides the timing or condition for the main future action (izgubit ćemo).

Is mora the verb meaning 'must' here?

No, it is a false friend in this context! Here, mora is the genitive case of the neuter noun more (sea). The phrase razina mora literally means 'level of the sea' or 'sea level'.

Why does the verb form rasla end in -a?

Because it must match the subject in gender and number. In Future II, the main verb is in the past participle form (the same one you use for the past tense). The subject of this clause is razina (level), which is a feminine singular noun, so the verb takes the feminine singular ending -la.

Why is the 'i' dropped from the end of the infinitive izgubit?

This is a strict spelling and pronunciation rule for Future I. Whenever an infinitive ending in -ti (like izgubiti) is placed immediately before the future auxiliary (ćemo), you must drop the final -i. If you reversed the word order, the -i would stay: mi ćemo izgubiti obalu.