What the heck is this thing

[Madreefer]

Aptasia.
Try to get rid of it. Inject with lemon juice or vinegar. You can but syringes from a drug store. Don't try scraping if off as it'll release a bunch of spores and infest your tank. Or get a peppermint shrimp. They're hit and miss too, some stores sell camelback shrimp as peppermint.