those are Staurocladia oahuensis or a hydroid jellyfish(crawling jellyfish) they do not get bigger then what you have, they are also unable to swim so the stick to the glass like a starfish and move around. They are said to have a sting but nothing is affected by them, there pretty harmless and will be there one day and gone the next, there population never really grows out of whack. the introduction of baby brine shrimp, cyclopeeze can encourage there growth and population for the simple fact they can feed on it. Really nothing to worry about i have dozens on my glass every time i turn my lights on. Lately i have been using oyster eggs so i am thinking that's what is increasing the population in my tank