Learn French: Je crois qu'elle signale ce compte. - I believe she is reporting this account.

While expressions of emotion and doubt (like aimer que) trigger the subjunctive, croire que in the affirmative expresses a strong belief or certainty. Therefore, it takes the normal indicative mood (elle signale). You would only use the subjunctive if the phrase were negative (Je ne crois pas qu'elle signale...) or a question, because those introduce doubt.

I know 'compte' from banking. What kind of account is being reported here?

While un compte can indeed mean a bank account, the verb signaler (to report or flag) tells us this is likely about the internet. In this context, she is reporting a user's social media account.

The English translation uses 'is reporting'. Why is there no form of the verb 'être' (to be) in the French sentence?

French does not have a separate present continuous tense (like 'am doing' or 'is reporting'). The simple present tense elle signale covers both 'she reports' in general and 'she is reporting' right now.

Could I use 'cet' instead of 'ce' before 'compte'?

No, because compte is a masculine noun that starts with a consonant. You use ce for masculine words starting with a consonant, cet for masculine words starting with a vowel sound (like cet ami), and cette for all feminine words.