Yes they can. Normal Flossing, Brushing, and WaterPik's should be able to remove them efficiently. If they do not, it would be best to consult with a dentist in person to see if they can help.

Not sure about the question, food under the crown of the implant or the actual implant. If it's under the crown, new crown will be needed. If you mean under the actual implant, the implant has failed. Either way, contact your dentist ASAP.