1. Direct conflict of interest

Trump publicly promotes an action (DJT) of his company while he is in office. This is a clear case of conflict of interest: he is potentially using his political power to influence the market to his advantage.

2. Market manipulation

Although presidents have freedom of speech, if a tweet causes price movements on an asset owned directly or indirectly, it brushes against or enters the realm of market manipulation, especially when combined with insider trading or personal benefits.

3. Violation of the Presidential Ethics Act (public ethics)

In the United States, the president is technically exempt from certain ethical laws that bind other officials, but:

he can still be investigated or challenged by Congress,

he can face enormous political pressure or even a formal motion for abuse of power.