Try learning what a real Ponzi scheme is. The Ponzi schemer takes your money on the pretext that he will invest it. He actually puts it in his own bank account. He pays early investors with money from more recent investors.
mREITs raise capital and invest it in mortgages. They try to make money on spreads. For several reasons, it`s risky but it is in no way, shape or form a Ponzi scheme.
By the way, all REITs must continuously raise capital to grow because they are required by law to pay out 90% of taxable income as dividends. Repeat: "Required by law".