As a green card holder, you are subject to U.S. tax on your worldwide income. Your U.S. tax obligation likely started in April, even though you did not have U.S. source income until recently. You need to check what your filing obligations are in Singapore. In 2017, any tax you pay to Singapore on income also taxable in the U.S. should be eligible for a foreign tax credit. Best for you to contact a U.S. tax professional.
