If Merrill sells anything in the Guided Investing account that has a capital gain, you'll be subject to capital gains taxes. But if it's in FDIC deposits, money market funds, ultra-short bond funds, or similar types of investments that generate income with minimal price movement, you'll likely have a relatively small tax impact.
The IRA won't have a taxable event if anything is sold, just be aware that if the transfer isn't a direct rollover from ML to Wealthfront, you'll have a mandatory 20% tax withholding and 60 days to complete the rollover.